slip: Use net_device_stats from struct net_device

Use net_device->stats for stats instead of private variable copies in
struct slip. Use ndo_get_stat64 so the additions can be performed on a private
destination buffer.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>