iio: adc: ad_sigma_delta: refactor setting read address

iio: adc: ad_sigma_delta: refactor setting read address

Refactor code to set the read address in a separate function.

This code is already duplicated twice and we will need to use it a third
time in a later commit.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250701-iio-adc-ad7173-add-spi-offload-support-v3-7-42abb83e3dac@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>