staging:iio: remove ability to escalate events.

Whilst it is possible to output events to say buffers have passed
a particular level there are no obvious reasons to actually do so.

The upshot of this patch is that buffers will only ever have
one threshold turned on at a time.

For now sca3000 has it's ring buffer effectively disabled.
Fixed later in series.

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