wifi: mt76: mt7915: call mt7915_mcu_set_thermal_throttling() only after init_work

Enable thermal management by default shall not be executed during mcu
init. This causes thermal configuration being reset to the firmware
default settings.

Fixes: 0063b86c ("mt76: mt7915e: Enable thermal management by default")
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>