net/mlx5: Support devices with more than 2 ports

Increase the define MLX5_MAX_PORTS to 4 as the driver is ready
to support NICs with 4 ports.

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