thermal: core: Use device_unregister() instead of device_del/put()

Lets not open code device_unregister() unnecessarily.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>