staging: comedi: pass subdevice to comedi_buf_munge()

staging: comedi: pass subdevice to comedi_buf_munge()

Local function `comedi_buf_munge()` is called by
`comedi_buf_write_free()` to modify (or "munge") the data copied from
a device into the buffer.

Currently, the function takes a pointer to a `struct comedi_async` and
gets a pointer to the comedi subdevice from the `subdevice` member.
Change it to take a pointer to a `struct comedi_subdevice` and get a
pointer to the "async" structure from the `async` member.

The main motivation for this is to elimate 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>