usb: gadget: rndis: init & exit rndis at module load/unload

This is required in preparation for using usb_gstrings_attach.

The rndis initialization so far has been performed on the first
occurence of rndis_bind(), but the condition to check it (first
or not first) was "borrowed" from strings handling.

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>