r8169: move switching optional clock on/off to pll power functions

Relevant chip clocks are disabled in rtl_pll_power_down(), therefore
move calling clk_disable_unprepare() there. Similar for enabling the
clock.

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