mt76: mt7615: fix hwmon temp sensor mem use-after-free

Without this change, garbage is seen in the hwmon name and sensors output
for mt7615 is garbled.

Fixes: 109e505a ("mt76: mt7615: add thermal sensor device support")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>