PCI PM: Simplify PCI wake-up code

Rework the PCI wake-up code so that it's easier to read without
changing the functionality.

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>