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>
diff --git a/drivers/net/usb/Kconfig b/drivers/net/usb/Kconfig
index 9f194a0..37eed4d 100644
--- a/drivers/net/usb/Kconfig
+++ b/drivers/net/usb/Kconfig
@@ -5,8 +5,8 @@
 	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