Review soft-channel device support's soft_init code

[kmpeters]

@MarkRivers noticed that the soft_init code is "needlessly complex": https://github.com/epics-modules/motor/issues/115#issuecomment-428353753

@timmmooney also has concerns: https://github.com/epics-modules/motor/issues/115#issuecomment-428745479

I agree that the code should be improved. It will be a priority after R7-0.