PCI: PM: Avoid resuming devices in D3hot during system suspend

The current code resumes devices in D3hot during system suspend if
the target power state for them is D3cold, but that is not necessary
in general.  It only is necessary to do that if the platform firmware
requires the device to be resumed, but that should be covered by
the platform_pci_need_resume() check anyway, so rework
pci_dev_keep_suspended() to avoid returning 'false' for devices
in D3hot which need not be resumed due to platform firmware
requirements.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>