r8169: improve runtime pm in general and suspend unused ports

So far rpm doesn't cover cases like unused ports which are never
brought up. If they are active at probe time they remain in this state.
Included in this patch:

- Let the idle notification check whether we can suspend and let it
  schedule the suspend. This way we don't need to have calls to
  pm_schedule_suspend in different places.

- At the end of rtl_open and rtl_init_one send an idle notification
  to allow suspending if the link is down. If a cable is plugged in
  aneg is finished before the suspend timer expires and the suspend
  request is cancelled.

- Change rtl8169_runtime_suspend to power down the chip if the
  interface is down.

Successfully tested on a RTL8168evl (mac version 34).

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