iio: adc: ad9467: support multiple channels calibration

iio: adc: ad9467: support multiple channels calibration

The calibration process mixes the support for calibrating multiple
channels with only having one channel. Some paths do have 'num_channels'
into account while others don't.

As of now, the driver only supports devices with one channel so the
above is not really a problem. That said, we'll add support for devices
with more than one channel, hence let's properly make the calibration
process to work with it.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240704-dev-iio-ad9467-new-devs-v1-1-f1adfee921f7@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>