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

Use netdev->tstats instead of a member of qtnf_vif for storing a pointer
to the per-cpu counters. This allows us to use core functionality for
statistics handling.
The driver sets netdev->needs_free_netdev, therefore freeing the per-cpu
counters at the right point in time is a little bit tricky. Best option
seems to be to use the ndo_init/ndo_uninit callbacks.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>