bridge: Use on-device stats instead of private ones.

Even though bridges require 6 fields from struct net_device_stats,
the on-device stats are always there, so we may just use them.

The br_dev_get_stats is no longer required after this.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>