iio: adc: ad7606: wrap channel ranges & scales into struct

With the addition of AD7606C-16,18 which have differential & bipolar
channels (and ranges), which can vary from channel to channel, we'll need
to keep more information about each channel range.

To do that, we'll add a 'struct ad7606_chan_scale' type to hold just
configuration for each channel.
This includes the scales per channel (which can be different with
AD7606C-16,18), as well as the range for each channel.
This driver was already keeping the range value for each channel before,
and since this is couple with the scales, it also makes sense to put them
in the same struct.

Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240919130444.2100447-5-aardelean@baylibre.com


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