net: eth: cpsw: Use net_device_stats from struct net_device

Instead of using an own copy of struct net_device_stats in struct
cpsw_priv, use stats from struct net_device. Also remove the thus
unnecessary .ndo_get_stats function, as it just returns dev->stats,
which is the default.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>