iio: event: NULL-ify IIO device's event_interface ref during unregister

Though we know that the iio_device_unregister_eventset() call is followed
by the free-ing of the IIO device object, we should not make this
assumption in the iio_device_unregister_eventset() function. It should
allow for the clean unregistering of the event-set, allowing a re-register
should we decide to implement this at some point later.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200921103156.194748-2-alexandru.ardelean@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-event.c

@@ -561,4 +561,5 @@ void iio_device_unregister_eventset(struct iio_dev *indio_dev)
 	iio_free_chan_devattr_list(&ev_int->dev_attr_list);
 	kfree(ev_int->group.attrs);
 	kfree(ev_int);
+	iio_dev_opaque->event_interface = NULL;
 }