iio: dummy: Use a fixed structure to build up scan to push to buffers.

It has long been discouraged for drivers to make use of iio_dev->scan_bytes
directly as that is an implementation detail of the core. As such our
example driver should definitely not be doing so.

In order to illustrate the more complex case, where a DMA safe buffer is
needed, continue to kzalloc() the storage (but with a structure definition
to provide an explicit data layout). Also add comments on when a DMA safe
buffer is necessary and the two common ways of obtaining one.

Whilst we have a mixture of signed and unsigned channels, the unsigned
channels have ranges that can be stored in a signed value - hence
use signed storage for all channels, simplifying the structure definition.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250413103443.2420727-7-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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