iio: buffer: use struct iio_scan_type to simplify code

By using struct iio_scan_type, we can simplify the code by removing
lots of duplicate pointer dereferences. This make the code a bit easier
to read.

This also prepares for a future where channels may have more than one
scan_type.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240530-iio-add-support-for-multiple-scan-types-v3-2-cbc4acea2cfa@baylibre.com


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