net: macsec: 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/0d81e0f7-7784-42df-8e10-d0b77ca5b7ee@gmail.com


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