net/mlx5: Lag, don't use magic numbers for ports

Instead of using 1 & 2 as the ports numbers use an enum value.

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