Reuse map_voltage() to get the selector of a given uV.

Then we can remove wm8350_ldo_mvolts_to_val() and wm8350_dcdc_mvolts_to_val().
Also remove unused wm8350_dcdc_val_to_mvolts() function.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>