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

Use netdev->tstats instead of a member of qmimux_priv 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>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>