net/mlx5: Lag, expose number of lag ports

Downstream patches will add support for hardware lag with
more than 2 ports. Add a way for users to query the number of lag ports.

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