net/mlx5: Lag, support single FDB only on 2 ports

E-Switch currently doesn't support more than 2 E-Switch managers
being aggregated under a single hardware lag. Have specific checks
to disallow creating lag when the code doesn't support it.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>