net: cxgb3: Use net_device_stats from struct net_device

Instead of using a private copy of struct net_device_stats in
struct port_info, use stats from struct net_device.

Cc: Santosh Raspatur <santosh@chelsio.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>