6pack: use netstats in net_device structure

Use net_device_stats from net_device structure instead of local.
Kill sp_get_stats function, because by default it is used identical
internal_stats function from net/core/dev.c

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>