r8169: add rtl_enable_exit_l1

This adds a function for what has been magic register writes so far.
It's based on recent changes to vendor drivers r8101, r8168, r8125,
and deals with events that trigger an early ASPM L1 exit.
Description of the bits has been kindly provided by Realtek.

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