Fix the following coccicheck warning:

drivers/net/ethernet/mellanox/mlx4/en_ethtool.c:1238:5-8: Unneeded
variable: "err". Return "0" on line 1252
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>