usb: gadget: f_rndis: fix an error code on allocation failure

This should be return -ENOMEM.  The current code returns successs.

Fixes: de7a8d2d534f ('usb: gadget: f_rndis: OS descriptors support')
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>