iio: adc: ad7173: refactor channel configuration parsing

iio: adc: ad7173: refactor channel configuration parsing

Move configurations regarding number of channels from
*_fw_parse_device_config to *_fw_parse_channel_config.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/all/20240303162148.3ad91aa2@jic23-huawei/
Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Link: https://patch.msgid.link/20240607-ad4111-v7-4-97e3855900a0@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>