net: fix USB network driver config option.

It must be tristate to avoid broken dependencies with kernel built-in
usb network drivers when usb support is module only.

When net config option is set, least surprize default should match usb.

Wireless RNDIS USB driver used to select USB_USBNET. USB_USBNET now
depends on USB_NET_DRIVERS so the latter should be selected as well.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/Kconfig

@@ -5,8 +5,8 @@ comment "Host-side USB support is needed for USB Network Adapter support"
 	depends on !USB && NET
 
 menuconfig USB_NET_DRIVERS
-	bool "USB Network Adapters"
-	default y
+	tristate "USB Network Adapters"
+	default USB if USB
 	depends on USB && NET
 
 if USB_NET_DRIVERS

drivers/net/wireless/Kconfig

@@ -189,6 +189,7 @@ config USB_NET_RNDIS_WLAN
 	tristate "Wireless RNDIS USB support"
 	depends on USB
 	depends on CFG80211
+	select USB_NET_DRIVERS
 	select USB_USBNET
 	select USB_NET_CDCETHER
 	select USB_NET_RNDIS_HOST