net: asix: remove code duplicates in asix_mdio_read/write and asix_mdio_read/write_nopm

This functions are mostly same except of one hard coded "in_pm" variable.
So, rework them to reduce maintenance overhead.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20220223110633.3006551-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>