usb: gadget: rndis: convert into module

In order to convert to configfs the usb functions need to be converted
to a new interface and compiled as modules. This patch creates an rndis
module which will be used by the new functions. After all users of
f_rndis are converted to the new interface, this module can be
merged with f_rndis module.

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