r8169: move rtl_wol_enable_rx() and rtl_prepare_power_down()

There is no functional change. Moving these two functions for following
patch "r8169: fix dmar pte write access is not set error".

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