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>