Timeout weirdness in mqtt_run loop

I noticed some weirdness in the `mqtt_run` function. Specifically:

1) in the condition of the while loop https://github.com/CHERIoT-Platform/network-stack/blob/a7314f938e6f7cbb3cf125d77383e71a6d0ca22f/lib/mqtt/mqtt.cc#L1115 I don't think `status != MQTT_Success` is ever true because if it were we would've hit one of the return statements above. This means the loop always executes exactly once, so what's the point of having a loop?! For the same reason I don't think the return at https://github.com/CHERIoT-Platform/network-stack/blob/a7314f938e6f7cbb3cf125d77383e71a6d0ca22f/lib/mqtt/mqtt.cc#L1119 is ever reachable so the function will never return `-ETIMEDOUT`.

2) Relatedly, although the function takes a Timeout and elapses it, it will call `MQTT_ProcessLoop` (which I presume may block?) even if `timeout->may_block()` is false. Is this the correct behaviour?

I noticed this because this function is called in a loop in the Hugh demo [here](https://github.com/CHERIoT-Platform/cheriot-demos/blob/755fc02598dd71c39a3ddc0f5198db0d0307a49d/HughTheLightbulbV2/Device/mqtt.cc#L234) and I noticed that it never timed out in long runs even though the timeout was never reset. This is harmless in this case and actually what we want, but it's weird.