Updating _uStepCount when microstepping is changed

[Zhi-en]

The purpose of _uStepCount is to maintain 200 steps/rev even at microstepping of 256. However because this _uStepCount is fixed at 256, when the microstepping is changed via chopconf mres, the steps/rev changes. For example when mres is 1 (microstepping 128), it becomes 100steps/rev, because _uStepCount is still 256, not 128.

Would like to request for _uStepCount to be updated whenever mres is updated, such that all the functions: getCurrentSpeed, setMaxSpeed, getCurrentPosition, setTargetPosition, etc. would all continue to work on the 200 steps/rev logic.