hwmon: (sht15) Use devm_regulator_get_optional()

Since the sht15 driver supports operation without an external vref
regulator the driver should use the new devm_regulator_get_optional() to
indicate that a stub regulator should not be provided.
Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/sht15.c

@@ -957,7 +957,7 @@ static int sht15_probe(struct platform_device *pdev)
 	 * If a regulator is available,
 	 * query what the supply voltage actually is!
 	 */
-	data->reg = devm_regulator_get(data->dev, "vcc");
+	data->reg = devm_regulator_get_optional(data->dev, "vcc");
 	if (!IS_ERR(data->reg)) {
 		int voltage;