r8169: enable ALDPS for power saving

Enable ALDPS function to save power when link down. Note that the
feature should be set after the other PHY settings. And the firmware
is necessary. Don't enable it without loading the firmware.

None of the firmware-free chipsets support ALDPS. Neither do the
RTL8168d/8111d.

For 8136 series, make sure the ALDPS is disabled before loading the
firmware. For 8168 series, the ALDPS would be disabled automatically
when loading firmware. You must not disable it directly.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>