regulator: core: Call set_voltage_time_sel() only when the regulator is on

If the regulator is not on, it won't take time setting new voltage.
So only call set_voltage_time_sel() to get the necessary delay when
the regulator is on.

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