accel: ethosu: Ensure cmd stream ends with a stop op

In the Linux kernel, the following vulnerability has been resolved: accel: ethosu: Ensure cmd stream ends with a stop op While the QSIZE register setting should prevent an out of bounds access of the command stream, it is not clear whether the h/w generates an interrupt in this case as is required (to prevent a timeout). As a stop op is expected end of the command stream, let's just ensure it is present. A stop op in the middle of the command stream also makes no sense.