USB: gadget: legacy: fix an error code in eth_bind()

This code should return -ENOMEM if the allocation fails but it currently
returns success.

Fixes: 9b95236e

 ("usb: gadget: ether: allocate and init otg descriptor by otg capabilities")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YBKE9rqVuJEOUWpW@mwanda
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>