net/mlx5: Correct comment for legacy fields

fdb_table is used for both legacy and offloads mode.
It was incorrect to comment that fdb_table is legacy specific.
Hence, fix the comment to reflect that fdb_table is used in legacy and
offloads mode.

Fixes: 131ce701 ("net/mlx5: E-Switch, Remove redundant mc_promisc NULL check")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Vu Pham <vuhuong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>