thermal: core: Make thermal_zone_device_unregister() return after freeing the zone

Make thermal_zone_device_unregister() wait until all of the references
to the given thermal zone object have been dropped and free it before
returning.

This guarantees that when thermal_zone_device_unregister() returns,
there is no leftover activity regarding the thermal zone in question
which is required by some of its callers (for instance, modular driver
code that wants to know when it is safe to let the module go away).

Subsequently, this will allow some confusing device_is_registered()
checks to be dropped from the thermal sysfs and core code.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-and-tested-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>