r8169: clean up rtl_pll_power_down/up functions

Clean up the remainings of rtl_pll_power_down/up and rename
rtl_pll_power_down() to rtl_prepare_power_down().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>