r8169: use the generic EEE management functions

Now that the Realtek PHY driver maps the vendor-specific EEE registers
to the standard MMD registers, we can remove all special handling and
use the generic functions phy_ethtool_get/set_eee.

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