staging:iio: Make sure all triggers have a trigger_ops for the owner field.

The core needs the owner field to prevent module removal whilst in use and
uses it without confirming that the trigger_ops structure actually exists.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>