wifi: mt76: mt7915: check devm_kasprintf() returned value

wifi: mt76: mt7915: check devm_kasprintf() returned value

devm_kasprintf() can return a NULL pointer on failure but this returned
value is not checked. Fix this lack and check the returned value.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 6ae39b7c7ed4 ("wifi: mt76: mt7921: Support temp sensor")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://patch.msgid.link/20240903014955.4145423-1-make24@iscas.ac.cn
Signed-off-by: Felix Fietkau <nbd@nbd.name>