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.mountain
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>