iio: adc: ti-ads1100: fix error code in probe()

This code has a copy and paste bug so it accidentally returns
"PTR_ERR(data->reg_vdd)" which is a valid pointer cast to int. It
should return "ret" instead.

Fixes: 54188054

 ("iio: adc: Add TI ADS1100 and ADS1000")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/36fa2aeb-f392-4793-8b38-ae15514033c8@kili.mountain


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