usb: gadget: f_rndis: fix interface id for OS descriptors

f->os_desc_table[0].if_id is zero by default. If the actual id happens
to be different then no Feature Descriptors will be returned to the host
for this interface, so assign if_id as soon as it is known.

Cc: <stable@vger.kernel.org> # v3.16
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>