set pump speed to 0 when channel is inactive

[lawther]

Currently when the pump is switched off via turning the channel off - its last reported speed sticks. This is because we only update the pump speed when the pump reports its speed. But the pump will never report '0', because it has been powered off.

When the Channel is switched off, thus cutting power to the pump, we should set the pump speed to zero.

[marklynch]

That's a good point - do you want to add a PR for this and I'll review

[lawther]

Yep, I can do that. I'll try to get to it this week.