thermal/core: Correct function name thermal_zone_device_unregister()

Fix the following make W=1 kernel build warning:

  drivers/thermal/thermal_core.c:1376: warning: expecting prototype for thermal_device_unregister(). Prototype was for thermal_zone_device_unregister() instead

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210517051020.3463536-1-yangyingliang@huawei.com