usb: get rid of 'choice' for legacy gadget drivers

drivers/usb/gadget/legacy/Kconfig creates a 'choice' inside another
'choice'.

The outer choice: line 17 "USB Gadget precomposed configurations"
The inner choice: line 484 "EHCI Debug Device mode"

I wondered why the whole legacy gadget drivers reside in such a big
choice block.

This dates back to 2003, "[PATCH] USB: fix for multiple definition of
`usb_gadget_get_string'". [1]

At that time, the global function, usb_gadget_get_string(), was linked
into multiple drivers. That was why only one driver was able to become
built-in at the same time.

Later, commit a84d9e53 ("usb: gadget: start with libcomposite")
moved usb_gadget_get_string() to a separate module, libcomposite.ko
instead of including usbstring.c from multiple modules.

More and more refactoring was done, and after commit 1bcce939
("usb: gadget: multi: convert to new interface of f_mass_storage"),
you can link multiple gadget drivers into vmlinux without causing
multiple definition error.

This is the only user of the nested choice structure ever. Removing
this mess will make some Kconfig cleanups possible.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=fee4cf49a81381e072c063571d1aadbb29207408Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Felipe Balbi <balbi@kernel.org>
Link: https://lore.kernel.org/r/20200315154948.26569-1-masahiroy@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/legacy/Kconfig

@@ -13,32 +13,28 @@
 # With help from a special transceiver and a "Mini-AB" jack, systems with
 # both kinds of controller can also support "USB On-the-Go" (CONFIG_USB_OTG).
 #
+# A Linux "Gadget Driver" talks to the USB Peripheral Controller
+# driver through the abstract "gadget" API.  Some other operating
+# systems call these "client" drivers, of which "class drivers"
+# are a subset (implementing a USB device class specification).
+# A gadget driver implements one or more USB functions using
+# the peripheral hardware.
+#
+# Gadget drivers are hardware-neutral, or "platform independent",
+# except that they sometimes must understand quirks or limitations
+# of the particular controllers they work with.  For example, when
+# a controller doesn't support alternate configurations or provide
+# enough of the right types of endpoints, the gadget driver might
+# not be able work with that controller, or might need to implement
+# a less common variant of a device class protocol.
+#
+# The available choices each represent a single precomposed USB
+# gadget configuration. In the device model, each option contains
+# both the device instantiation as a child for a USB gadget
+# controller, and the relevant drivers for each function declared
+# by the device.
 
-choice
-	tristate "USB Gadget precomposed configurations"
-	default USB_ETH
-	optional
-	help
-	  A Linux "Gadget Driver" talks to the USB Peripheral Controller
-	  driver through the abstract "gadget" API.  Some other operating
-	  systems call these "client" drivers, of which "class drivers"
-	  are a subset (implementing a USB device class specification).
-	  A gadget driver implements one or more USB functions using
-	  the peripheral hardware.
-
-	  Gadget drivers are hardware-neutral, or "platform independent",
-	  except that they sometimes must understand quirks or limitations
-	  of the particular controllers they work with.  For example, when
-	  a controller doesn't support alternate configurations or provide
-	  enough of the right types of endpoints, the gadget driver might
-	  not be able work with that controller, or might need to implement
-	  a less common variant of a device class protocol.
-
-	  The available choices each represent a single precomposed USB
-	  gadget configuration. In the device model, each option contains
-	  both the device instantiation as a child for a USB gadget
-	  controller, and the relevant drivers for each function declared
-	  by the device.
+menu "USB Gadget precomposed configurations"
 
 config USB_ZERO
 	tristate "Gadget Zero (DEVELOPMENT)"
@@ -527,4 +523,4 @@ config USB_RAW_GADGET
 	  Say "y" to link the driver statically, or "m" to build a
 	  dynamically linked module called "raw_gadget".
 
-endchoice
+endmenu