iio:trigger: Remove necessity to have a trig->ops structure.

There are a few cases where none of the callbacks are supplied and the
ops structure purely existed to provide the driver module.

Given that is done differently now, we don't need to have a trig_ops structure.
Allow for it not being there required a few additional sanity checks when
trying check if particular callbacks are set.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>