iio: adc: ti-ads1298: Fix error code in probe()

There is a copy and paste bug here, it should be "reg_vref" instead of
"reg_avdd".  The "priv->reg_avdd" variable is zero so it ends up
returning success.

Fixes: 00ef7708 ("iio: adc: ti-ads1298: Add driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Mike Looijmans <mike.looijmans@topic.nl>
Link: https://lore.kernel.org/r/5f393a87-ca8b-4e68-a6f4-a79f75a91ccb@moroto.mountainSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads1298.c

@@ -657,7 +657,7 @@ static int ads1298_probe(struct spi_device *spi)
 	priv->reg_vref = devm_regulator_get_optional(dev, "vref");
 	if (IS_ERR(priv->reg_vref)) {
 		if (PTR_ERR(priv->reg_vref) != -ENODEV)
-			return dev_err_probe(dev, PTR_ERR(priv->reg_avdd),
+			return dev_err_probe(dev, PTR_ERR(priv->reg_vref),
 					     "Failed to get vref regulator\n");
 
 		priv->reg_vref = NULL;