r8169: enable ASPM L1.2 if system vendor flags it as safe

On some systems there are compatibility issues with ASPM L1.2 and
RTL8125, therefore this state is disabled per default. To allow for
the L1.2 power saving on not affected systems, Realtek provides
vendors that successfully tested ASPM L1.2 the option to flag this
state as safe. According to Realtek this flag will be set first on
certain Chromebox devices.

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