staging: comedi: mite: pass subdevice to mite_buf_change()

staging: comedi: mite: pass subdevice to mite_buf_change()

`mite_buf_change()` in the "mite" module currently takes a pointer to a
`struct comedi_async` and gets a pointer to the owning `struct
comedi_subdevice` from the `subdevice` member.  Change it to take a
pointer to a `struct comedi_subdevice` and use the `async` member.

The motivation for this is to eliminate the `subdevice` member of
`struct comedi_async`.

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>