net/mlx5: Lag, store number of ports inside lag object

Store the number of lag ports inside the lag object. Lag object is a single
shared object managing the lag state of multiple mlx5 devices on the same
physical HCA.

Downstream patches will allow hardware lag to be created over devices with
more than 2 ports.

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