mac80211: 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>
Link: https://lore.kernel.org/r/93dda477-70ae-0ccf-71b4-bfebb66c9beb@gmail.com


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