PCI PM: Fix pci_prepare_to_sleep

The recently introduced pci_prepare_to_sleep() needs the following fix,
because there are systems which are not power manageable by ACPI (ie.
ACPI doesn't provide methods to put the device into low power states and
back), but require ACPI hooks to be executed for wake-up to work.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>