iio: ti-adc12138: Use scan_type when processing raw data

Use channel definition as root of trust and replace constant
when reading elements directly using the raw sysfs attributes.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20211104082413.3681212-10-gwendal@chromium.org


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