Convert ab8500 to set_voltage_sel and then we can remove
ab8500_get_best_voltage_index function.
Signed-off-by: Axel Lin <axel.lin@gmail.com> Tested-by: Linus Walleij <linus.walleij@linaro.org> Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>