iio: adc: ad7606: Introduce num_adc_channels

This variable determines how many hardware channels has the chip,
oppositely to the num_channels that can contain more channels, e.g a
timestamp channel in our case. Introducing this variable avoids
decreasing the former num_channels variable when reading the ADC's
channels, and clarifies a bit the code.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-6-654faf1ae08c@baylibre.com


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