regulator: Fix _regulator_get_voltage if get_voltage callback is NULL

In the case of get_voltage callback is NULL, current implementation in
_regulator_get_voltage will return -EINVAL.

Also returns proper error if ret is negative value.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>

drivers/regulator/core.c

@@ -1886,12 +1886,14 @@ static int _regulator_get_voltage(struct regulator_dev *rdev)
 		if (sel < 0)
 			return sel;
 		ret = rdev->desc->ops->list_voltage(rdev, sel);
-	}
-	if (rdev->desc->ops->get_voltage)
+	} else if (rdev->desc->ops->get_voltage) {
 		ret = rdev->desc->ops->get_voltage(rdev);
-	else
+	} else {
 		return -EINVAL;
+	}
 
+	if (ret < 0)
+		return ret;
 	return ret - rdev->constraints->uV_offset;
 }