iio: introduce struct iio_scan_type

This gives the channel scan_type a named type so that it can be used
to simplify code in later commits.

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-v3-1-cbc4acea2cfa@baylibre.com


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