r8169 / PCI / PM: Add simplified runtime PM support (rev. 3)

Use the PCI runtime power management framework to add basic PCI
runtime PM support to the r8169 driver.  Namely, make the driver
suspend the device when the link is not present and set it up for
generating a wakeup event after the link has been detected again.
[This feature is disabled until the user space enables it with the
help of the /sys/devices/.../power/contol device attribute.]

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>