Use net_device_stats from net_device structure instead of local.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Tested-by: Marc Pignat <marc.pignat@hevs.ch>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>