regulator: Make s5m8767_get_voltage_register always return correct register

Check s5m8767->buck[2|3|4]_gpiodvs status in s5m8767_get_voltage_register
and return correct register accordingly.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>