regulator: core: Fix continuous_voltage_range case in regulator_can_change_voltage

Regulator drivers with continuous_voltage_range flag set allows not setting
n_voltages. Thus if continuous_voltage_range is set, check the constraint range
instead.

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