staging:iio:triggers introduce iio_trigger_ops to take const bits out of iio_trig_structure.

Right now this results in increased code, but I still think it is worth doing to avoid
replication across instances of drivers etc and move as much stuff as possible to constant.
Ops structure is optional for the occasional driver that uses none of it (currently
only the ad7793).

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