regulator: Convert ab8499 to use get_voltage_sel()

This change is required to make ab8500_regulator_get_voltage_sel work.
The regulator core will call set_voltage_time_sel only when get_voltage_sel is
implemented.

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>