staging:iio:adc:ad7816 move from old to current event handling.

I'm not sure what the event is and am very much against the dodgy
hack to give it a code. However for now, lets just stop it using
the deprecated core handling so we can move on.

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