r8169: add EEE support for RTL8168f

Add EEE support for RTL8168f to the recently added EEE handling
framework in the driver. This patch leaves the chip defaults, means
EEE typically is disabled initially and it's up to the user to enable
it via ethtool.

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