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>
                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;
        }