net: usb: qmi_wwan: 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: Bjørn Mork <bjorn@mork.no>
Link: https://lore.kernel.org/r/2c97b75b-107e-0ab6-d9ef-9f38bb03f495@gmail.com


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