iio: ad5686: fix optional reference voltage declaration

commit da019f59 upstream.

When not using the "_optional" function, a dummy regulator is returned
and the driver fails to initialize.
Signed-off-by: Urs Fässler <urs.fassler@bytesatwork.ch>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/iio/dac/ad5686.c

@@ -322,7 +322,7 @@ static int ad5686_probe(struct spi_device *spi)
 	st = iio_priv(indio_dev);
 	spi_set_drvdata(spi, indio_dev);
 
-	st->reg = devm_regulator_get(&spi->dev, "vcc");
+	st->reg = devm_regulator_get_optional(&spi->dev, "vcc");
 	if (!IS_ERR(st->reg)) {
 		ret = regulator_enable(st->reg);
 		if (ret)