iio: adc: extract setup function without backend

Refactor probe function by moving the initialization specific to
communication without iio-backend into a separate setup function.

The purpose of this modification is better code organization. No
functional changes intended.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Ioana Risteiu <Ioana.Risteiu@analog.com>
Link: https://patch.msgid.link/20250825221355.6214-4-Ioana.Risteiu@analog.com


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