regulator: Use tps65912_set_voltage_sel for both DCDCx and LDOx

regulator: Use tps65912_set_voltage_sel for both DCDCx and LDOx

commit 42b5ef "regulator: tps65912: Use simple equations to get register address"
uses tps65912_get_sel_register to replace tps65912_get_dcdc_sel_register
and tps65912_get_ldo_sel_register.

Now tps65912_set_voltage_dcdc_sel and tps65912_set_voltage_ldo_sel has exactly
the same implementation. Merge them to tps65912_set_voltage_sel function.

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