r8169: improve rtl_set_d3_pll_down

Make use of new helper r8169_mod_reg8_cond() and move from a switch()
to an if() clause. Benefit is that we don't have to touch this piece of
code each time support for a new chip version is added.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/e1ccdb85-a4ed-4800-89c2-89770ff06452@gmail.com


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