thermal: Prevent polling from happening during system suspend

The thermal driver should use a freezable workqueue to schedule
polling to prevent thermal_zone_device_update() from being run
during system suspend, when the devices it relies on may be inactive.
Make it use the system freezable workqueue for this purpose.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>