iio: max1363: Use devm_regulator_get_optional for optional regulator

In kernel version 3.13, devm_regulator_get() may return no error
if a regulator is undeclared. regulator_get_voltage() will return
-EINVAL if this happens. This causes the driver to fail loading if
the vref regulator is not declared.

Since vref is optional, call devm_regulator_get_optional instead.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/max1363.c

@@ -1560,7 +1560,7 @@ static int max1363_probe(struct i2c_client *client,
 	st->client = client;
 
 	st->vref_uv = st->chip_info->int_vref_mv * 1000;
-	vref = devm_regulator_get(&client->dev, "vref");
+	vref = devm_regulator_get_optional(&client->dev, "vref");
 	if (!IS_ERR(vref)) {
 		int vref_uv;