wifi: mt76: mt7915: add error message in mt7915_thermal_set_cur_throttle_state()

Add dev_err so that it is easier to see invalid setting while looking at
dmesg.

Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-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>