USB: cdc_subset: only build when one driver is enabled

This avoids a harmless randconfig warning I get when USB_NET_CDC_SUBSET
is enabled, but all of the more specific drivers are not:

drivers/net/usb/cdc_subset.c:241:2: #warning You need to configure some hardware for this driver

The current behavior is clearly intentional, giving a warning when
a user picks a configuration that won't do anything good. The only
reason for even addressing this is that I'm getting close to
eliminating all 'randconfig' warnings on ARM, and this came up
a couple of times.

My workaround is to not even build the module when none of the
configurations are enable.

Alternatively we could simply remove the #warning (nothing wrong
for compile-testing), turn it into a runtime warning, or
change the Kconfig options into a menu to hide CONFIG_USB_NET_CDC_SUBSET.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/Kconfig

@@ -395,6 +395,10 @@ config USB_NET_RNDIS_HOST
 	  The protocol specification is incomplete, and is controlled by
 	  (and for) Microsoft; it isn't an "Open" ecosystem or market.
 
+config USB_NET_CDC_SUBSET_ENABLE
+	tristate
+	depends on USB_NET_CDC_SUBSET
+
 config USB_NET_CDC_SUBSET
 	tristate "Simple USB Network Links (CDC Ethernet subset)"
 	depends on USB_USBNET
@@ -413,6 +417,7 @@ config USB_NET_CDC_SUBSET
 config USB_ALI_M5632
 	bool "ALi M5632 based 'USB 2.0 Data Link' cables"
 	depends on USB_NET_CDC_SUBSET
+	select USB_NET_CDC_SUBSET_ENABLE
 	help
 	  Choose this option if you're using a host-to-host cable
 	  based on this design, which supports USB 2.0 high speed.
@@ -420,6 +425,7 @@ config USB_ALI_M5632
 config USB_AN2720
 	bool "AnchorChips 2720 based cables (Xircom PGUNET, ...)"
 	depends on USB_NET_CDC_SUBSET
+	select USB_NET_CDC_SUBSET_ENABLE
 	help
 	  Choose this option if you're using a host-to-host cable
 	  based on this design.  Note that AnchorChips is now a
@@ -428,6 +434,7 @@ config USB_AN2720
 config USB_BELKIN
 	bool "eTEK based host-to-host cables (Advance, Belkin, ...)"
 	depends on USB_NET_CDC_SUBSET
+	select USB_NET_CDC_SUBSET_ENABLE
 	default y
 	help
 	  Choose this option if you're using a host-to-host cable
@@ -437,6 +444,7 @@ config USB_BELKIN
 config USB_ARMLINUX
 	bool "Embedded ARM Linux links (iPaq, ...)"
 	depends on USB_NET_CDC_SUBSET
+	select USB_NET_CDC_SUBSET_ENABLE
 	default y
 	help
 	  Choose this option to support the "usb-eth" networking driver
@@ -454,6 +462,7 @@ config USB_ARMLINUX
 config USB_EPSON2888
 	bool "Epson 2888 based firmware (DEVELOPMENT)"
 	depends on USB_NET_CDC_SUBSET
+	select USB_NET_CDC_SUBSET_ENABLE
 	help
 	  Choose this option to support the usb networking links used
 	  by some sample firmware from Epson.
@@ -461,6 +470,7 @@ config USB_EPSON2888
 config USB_KC2190
 	bool "KT Technology KC2190 based cables (InstaNet)"
 	depends on USB_NET_CDC_SUBSET
+	select USB_NET_CDC_SUBSET_ENABLE
 	help
 	  Choose this option if you're using a host-to-host cable
 	  with one of these chips.

drivers/net/usb/Makefile

@@ -23,7 +23,7 @@ obj-$(CONFIG_USB_NET_GL620A)	+= gl620a.o
 obj-$(CONFIG_USB_NET_NET1080)	+= net1080.o
 obj-$(CONFIG_USB_NET_PLUSB)	+= plusb.o
 obj-$(CONFIG_USB_NET_RNDIS_HOST)	+= rndis_host.o
-obj-$(CONFIG_USB_NET_CDC_SUBSET)	+= cdc_subset.o
+obj-$(CONFIG_USB_NET_CDC_SUBSET_ENABLE)	+= cdc_subset.o
 obj-$(CONFIG_USB_NET_ZAURUS)	+= zaurus.o
 obj-$(CONFIG_USB_NET_MCS7830)	+= mcs7830.o
 obj-$(CONFIG_USB_USBNET)	+= usbnet.o