am79c961a: Use net_device_stats from struct net_device

struct net_device has its own struct net_device_stats member, so we can
use this one instead of a private copy in the dev_priv struct.

Cc: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>