r8169: merge r810x_pll_power_down/up into r8168_pll_power_down/up

r810x_pll_power_down/up and r8168_pll_power_down/up have a lot in common,
so we can simplify the code by merging the former into the latter.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>