NIMROD: excessive logging of motor - encoder difference

[FreddieAkeroyd]

The changes in #4449 were intended to only log the difference at the end of a move, due to the noisy encoder on NIMROD the (encoder - motor) value is changing when DMOV == 1 and being logged. To reduce this we could:

• only calculate a difference when moving (DMOV == 0), then record as usual (DMOV == 1)
• it might be possible to use a deadband

[Tom-Willemsen]

No longer seems to be logging large amounts on NIMROD.