regulator: core: Optimize _regulator_do_set_voltage if voltage does not change

Optimize _regulator_do_set_voltage() for the case selector is equal to
old_selector. Since the voltage does not change, we don't need to call
set_voltage_sel() and set_voltage_time_sel() in this case.

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