regulator: Make max8997_get_voltage_register always return correct register

Check max8997->buck[1|2|5]_gpiodvs status in max8997_get_voltage_register
and return correct register accordingly.

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