qtnfmac: 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>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/166259f2-084c-45d7-e610-2de2a0bdae06@gmail.com


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