regulator: core: Use list_voltage() to read single voltage regulators

If the regulator doesn't supply a way of reading back the voltage but does
provide a list_voltage() operation then use that with a selector of zero
to read the voltage.  Regulators doing this means that we have the list
operation there for consumers that want to configure themselves.
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/core.c

@@ -2395,6 +2395,8 @@ static int _regulator_get_voltage(struct regulator_dev *rdev)
 		ret = rdev->desc->ops->list_voltage(rdev, sel);
 	} else if (rdev->desc->ops->get_voltage) {
 		ret = rdev->desc->ops->get_voltage(rdev);
+	} else if (rdev->desc->ops->list_voltage) {
+		ret = rdev->desc->ops->list_voltage(rdev, 0);
 	} else {
 		return -EINVAL;
 	}