regulator: ab3100: Use regulator_map_voltage_iterate()

regulator_map_voltage_iterate() is for drivers implementing set_voltage_sel()
and list_voltage() to use it as their map_voltage() operation.

In this case, regulator_map_voltage_iterate() happen to be doing the same thing
as ab3100_get_best_voltage_index() function. So we can use it to replace
ab3100_get_best_voltage_index() function.

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