net: usbnet: 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/70ad3e33-8ea6-e12e-31de-5fec7a3c4f6e@gmail.com


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