net: dsa: use net core stats64 handling

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

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>