thermal: mt8173: minor mtk_thermal.c cleanups

If thermal bank with 4 sensors, thermal driver should read TEMP_MSR3.

However, currently thermal driver would not read TEMP_MSR3 since mt8173
thermal driver only use 3 sensors on each thermal bank at the same time,
so this patch would not effect temperature.
Only if mt mt8173 thermal driver use 4 sensors on any thermal bank, would
read third sensor two times, and lose fourth sensor of vale.

cc: stable@vger.kernel.org
Fixes: b7cf0053 ("thermal: Add Mediatek thermal driver for mt2701.")
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Dawei Chien <dawei.chien@mediatek.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>