Some devices require run-time axis calibration. We can't change the min/max
ranges once we've initialised the valuator structs though, so in-driver
run-time calibration is required.
If the property is set, the driver scales from the calibrated range to the
values reported to the X server (which then may scale to screen coordinates).
If the property is not set (i.e. zero items) no scaling is performed.
