regulator: Don't create voltage sysfs entries if we can't read voltage

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/core.c

@@ -2503,7 +2503,8 @@ static int add_regulator_attributes(struct regulator_dev *rdev)
 	int			status = 0;
 
 	/* some attributes need specific methods to be displayed */
-	if (ops->get_voltage || ops->get_voltage_sel) {
+	if ((ops->get_voltage && ops->get_voltage(rdev) >= 0) ||
+	    (ops->get_voltage_sel && ops->get_voltage_sel(rdev) >= 0)) {
 		status = device_create_file(dev, &dev_attr_microvolts);
 		if (status < 0)
 			return status;