net/mlx4_core: Change the error print to info print

The error print within mlx4_flow_steer_promisc_add() should
be a info print.

Fixes: 592e49dd ('net/mlx4: Implement promiscuous mode with device managed flow-steering')
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>