usbnet: switch to core handling of rx/tx byte/packet counters

Use netdev->tstats instead of a member of usbnet for storing a pointer
to the per-cpu counters. This allows us to use core functionality for
statistics handling.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>