mlx4_en: use net_device dev_id to indicate port number

Today, there are no means to know which port of a hardware device a netdev
interface uses. struct net_device conatins a field, dev_id, that can be used
for that. Use this field to save the port number in ConnectX that is being used
by the net device; port numbers are zero based.
Signed-off-by: NEli Cohen <eli@mellanox.co.il>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/mlx4/en_netdev.c

@@ -961,6 +961,7 @@ int mlx4_en_init_netdev(struct mlx4_en_dev *mdev, int port,
 	}
 
 	SET_NETDEV_DEV(dev, &mdev->dev->pdev->dev);
+	dev->dev_id =  port - 1;
 
 	/*
 	 * Initialize driver private data