usb: gadget: rndis: merge u_rndis.ko with usb_f_rndis.ko

The rndis function's users use only the new interface, so the two modules
can be merged.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>