regulator: Convert pcf50633 to get_voltage_sel

Convert pcf50633 to get_voltage_sel and then we can remove
pcf50633_regulator_voltage_value function and move its implementation
to pcf50633_regulator_list_voltage.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>