net/mlx5e: E-Switch, Add a check that log_max_l2_table is valid

If log_max_l2_table is 0 there is no really room for one L2 address.
and should be treated as not supported.
Do the check in MPFS init and for vport context events which
both used to update L2 address.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>