r8169: improve __rtl8169_set_wol

Add helper r8169_mod_reg8_cond() what allows to significantly simplify
__rtl8169_set_wol().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/697b197a-8eac-40c6-8847-27093cacec36@gmail.com


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