r8169: factor out disabling ALDPS

Chip versions from RTL8168g onward use the same sequence to disable
ALDPS (Advanced Link-Down Power Saving). So let's factor this out.

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