staging:iio:sca3000 extract old event handling and move to poll for events from buffer

Fairly substantial rewrite as the code had bitrotted.
A rethink is needed for how to handle variable types in the new chan_spec world.

This patch restores sca3000 buffer usage to a working state.
V3: Rebase fixups.
V2: Move to new version of IIO_CHAN macro

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>