e1000e / PCI / PM: Add basic runtime PM support (rev. 4)

Use the PCI runtime power management framework to add basic PCI
runtime PM support to the e1000e driver.  Namely, make the driver
suspend the device when the link is off 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.]

Based on a patch from Matthew Garrett.

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