ariadne: 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>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>