regulator: tps65910: Correct VIO voltage configuration

The VIO regulator register specify the voltage configuration
on bit3:2 of its register. And hence only these bits should
be modified when setting voltage and used when reading voltage
from register setting.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>