r8169: add general EEE support for chip versions from RTL8168g

This patch adds the general framework to deal with EEE in this driver
plus EEE support for chip versions from RTL8168g. We don't touch the
default chip settings, therefore EEE will usually be disabled and it's
up to the user to enable it.

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