The update is not immediate, we need to monitor UPD_DONE to be reassured
that values are applied and we can continue.

If this does not happen, then it is a serious hardware problem. That's
why I chose to use WARN_ON.

By returning and error code, we will get a ZIO_ALARM_LOST_TRIGGER, that
userspace needs to check.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>