regulator: twl: Convert twl4030ldo_ops to get_voltage_sel

This fixes an inconsistent behavior between list_voltage() and get_voltage()
because current implementation of get_voltage() does not check the case
IS_UNSUP() is true.

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