regulator: fixed: fix regulator_list_voltage() for regression

Commit c368e5f "regulator: fixed:
get rid of {get|list}_voltage()" broke regulator_list_voltage() for
the fixed regulator, because an earlier commit
5a52360 "regulator: core: provide
fixed voltage in desc for single voltage rail" missed to add support
for the fixed-voltage special case to that function. This patch
fixes that regression.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/regulator/core.c

@@ -2184,6 +2184,9 @@ int regulator_list_voltage(struct regulator *regulator, unsigned selector)
 	struct regulator_ops	*ops = rdev->desc->ops;
 	int			ret;
 
+	if (rdev->desc->fixed_uV && rdev->desc->n_voltages == 1 && !selector)
+		return rdev->desc->fixed_uV;
+
 	if (!ops->list_voltage || selector >= rdev->desc->n_voltages)
 		return -EINVAL;