staging:iio: Remove legacy event handling.

This requires all drivers using the channel registration code and
events to change in one go.

V3: remove unwanted irq enable from event handler.
V2: rebase related fixes to move to new IIO_CHAN macro. All trivial.

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