wifi: mt76: mt7996: Add NULL check in mt7996_thermal_init

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7996: Add NULL check in mt7996_thermal_init devm_kasprintf() can return a NULL pointer on failure,but this returned value in mt7996_thermal_init() is not checked. Add NULL check in mt7996_thermal_init(), to handle kernel NULL pointer dereference error.