PCI / PM: Do not resume any devices in pci_pm_prepare()

It should not be necessary to resume devices with ignore_children set
in pci_pm_prepare(), because they should be resumed explicitly by
their children drivers during suspend if need be and they will be
resumed by pci_pm_suspend() after that anyway, so avoid doing that.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>