3c59x: use netstats in net_device structure

Use net_device_stats from net_device structure instead of local.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Steffen Klassert <klassert@mathematik.tu-chemnitz.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>