The spec suggests that we should use _OST to notify the platform
about the status of notifications it sends us, for example so that
it doesn't repeate a notification that has been handled already.

This turns out to help reduce the amount of diagnostic output from
the ACPIPHP subsystem and speed up boot on at least one system that
generates multiple device check notifies for PCIe devices on the root
bus during boot.
Reported-and-tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>