wifi: mt76: mt7925: Fix null-ptr-deref in mt7925_thermal_init()

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7925: Fix null-ptr-deref in mt7925_thermal_init() devm_kasprintf() returns NULL on error. Currently, mt7925_thermal_init() does not check for this case, which results in a NULL pointer dereference. Add NULL check after devm_kasprintf() to prevent this issue.