staging:iio: remove broken support for multiple event interfaces.

We don't have a use case for these.  Two drivers appeared to use them
but both report all events on the first.

V2: Remove now irrelevant comment.
V3: Include fixup for adc/ad7280a.c

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>