thermal: mediatek: use of_device_get_match_data()

The usage of of_device_get_match_data() reduce the code size a bit.

Also, the only way to call mtk_thermal_probe() is to match an entry in
mtk_thermal_of_match[], so of_id cannot be NULL.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>