r8169: drop member pll_power_ops from struct rtl8169_private

After merging r810x_pll_power_down/up and r8168_pll_power_down/up we
don't need member pll_power_ops any longer and can drop it, thus
simplifying the code.

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