USB: gadget: u_ether: Fix a configfs return code

commit 650bf522 upstream.

If the string is invalid, this should return -EINVAL instead of 0.

Fixes: 73517cf4

 ("usb: gadget: add RNDIS configfs options for class/subclass/protocol")
Cc: stable <stable@vger.kernel.org>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YCqZ3P53yyIg5cn7@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>