staging:iio: Push interrupt setup down into the drivers for event lines.

It is much easier to do in driver, and the core does not add much.
Note all drivers will have to be updated with this patch.
None currently are.

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