regulator: tps51632: Fix writing to wrong register when enable_pwm_dvfs is set

When tps->enable_pwm_dvfs is true, write to TPS51632_VOLTAGE_BASE_REG rather
than TPS51632_VOLTAGE_SELECT_REG.

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