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: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>