wifi: mt76: mt7615: 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: 0bb4e918 ("mt76: mt7615: fix hwmon temp sensor mem use-after-free")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://patch.msgid.link/20240905014753.353271-1-make24@iscas.ac.cn


Signed-off-by: Felix Fietkau <nbd@nbd.name>