net/mlx5: E-Switch, Remove redundant mc_promisc NULL check

mc_promisc pointer points to an instance of struct esw_mc_addr allocated
as part of the esw structure.
Hence it cannot be NULL.
Removed such redundant check and assign where it is actually used.

While at it, add comment around legacy mode fields and move mc_promisc
close to other legacy mode structures to improve code redability.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>