PCI PM: Introduce device flag wakeup_prepared

Introduce a new PCI device flag, wakeup_prepared, to prevent PCI
wake-up preparation code from being executed twice in a row for the
same device and for the same purpose.

Reviewed-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>