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>