wifi: mt76: mt7915: rework mt7915_thermal_temp_store()

Call mt7915_mcu_set_thermal_protect() through
mt7915_thermal_temp_store() to update firmware trigger/restore temp
directly.

Fixes: 02ee68b9 ("mt76: mt7915: add control knobs for thermal throttling")
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>