iio: adc: ad7606: Fix hardcoded offset in the ADC channels

iio: adc: ad7606: Fix hardcoded offset in the ADC channels

When introducing num_adc_channels, I overlooked some new functions
created in a meanwhile that had also the hardcoded offset. This commit
adds the new logic to these functions.

Fixes: ef67f16e365c ("iio: adc: ad7606: Introduce num_adc_channels")
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241210-ad7606_add_iio_backend_software_mode-v2-1-6619c3e50d81@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>