wifi: mt76: mt7996: fix potential memory leakage when reading chip temperature

Without this commit, reading chip temperature will cause memory leakage.

Fixes: 6879b2e9 ("wifi: mt76: mt7996: add thermal sensor device support")
Reported-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>