staging: comedi: remove subdevice member of struct comedi_async

The `async` member of `struct comedi_subdevice` may point to a `struct
comedi_async` or may be NULL.  The `subdevice` member of `struct
comedi_async` points back to the `struct comedi_subdevice` associated
with it in a one-to-one relationship.

All uses of the `subdevice` member of `struct comedi_async` apart from
its initialization have now been removed (by passing around a pointer to
the subdevice instead of to the "async" structure), so get rid of it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedidev.h

@@ -108,8 +108,6 @@ struct comedi_buf_map {
 };
 
 struct comedi_async {
-	struct comedi_subdevice *subdevice;
-
 	void *prealloc_buf;	/* pre-allocated buffer */
 	unsigned int prealloc_bufsz;	/* buffer size, in bytes */
 	struct comedi_buf_map *buf_map;	/* map of buffer pages */

drivers/staging/comedi/drivers.c

@@ -307,7 +307,6 @@ static int __comedi_device_postconfig_async(struct comedi_device *dev,
 		return -ENOMEM;
 
 	init_waitqueue_head(&async->wait_head);
-	async->subdevice = s;
 	s->async = async;
 
 	async->max_bufsize = comedi_default_buf_maxsize_kb * 1024;