firmware: arm_scmi: Fix requested device removal race

In the Linux kernel, the following vulnerability has been resolved: firmware: arm_scmi: Fix requested device removal race scmi_protocol_device_unrequest() drops scmi_requested_devices_mtx while notifying listeners but continues to retain the per-protocol list head. When two SCMI drivers for the same protocol unregister concurrently, one thread can remove the final request and free the list head while the other is running its notifier. The latter then dereferences the freed list head after reacquiring the mutex and can free it a second time. Complete the list and IDR updates, including freeing an empty list head, before dropping the mutex. Keep the blocking notifier outside the critical section and retain only the detached request across the callback.