mlx4: attach multicast with correct flag

mlx4_multicast_attach/detach() should use always MLX4_MC_STEER flag

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.co.il>
Reviewed-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>