iio: adc: ad7173: refactor ain and vref selection

Move validation of analog inputs and reference voltage selection to
separate functions to reduce the size of the channel config parsing
function and improve readability.
Add defines for the number of analog inputs in a channel.

Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240607-ad4111-v7-5-97e3855900a0@analog.com


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