net: mlx4: remove unneeded variable "err" in mlx4_en_ethtool_add_mac_rule()

Fix the following coccicheck warning:

drivers/net/ethernet/mellanox/mlx4/en_ethtool.c:1396:5-8: Unneeded
variable: "err". Return "0" on line 1411

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>