wifi: mt76: mt7915: rework mt7915_mcu_set_thermal_throttling

This patch includes 2 changes:
1. Firmware expects to disable thermal protect first before
reconfiguring.
2. Separate setting thermal_protect and setting thermal_tx_duty into
different functions. These two firmware commands do not need to send
together.

Fixes: 34b877d9 ("mt76: mt7915: add thermal cooling device support")
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>