rdev->desc->uV_step * abs(new_selector - old_selector) returns uV.
The unit of ramp_delay is mV/us.

Current code multiples 1000 at wrong place.
Signed-off-by: NAxel Lin <axel.lin@gmail.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>