regulator: wm8400: Use wm8400_ldo_list_voltage instead of open code to verify selected voltage

Call wm8400_ldo_list_voltage() instead of open code to verify selected voltage
falls within specified range.
Use wm8400_ldo_list_voltage() here is less error prone.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>