net: dsa: use new function dev_fetch_sw_netstats

Simplify the code by using new function dev_fetch_sw_netstats().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/b6047017-8226-6b7e-a3cd-064e69fdfa27@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>