staging: comedi: pass subdevice to comedi_buf_write_n_available()

Local function `comedi_buf_write_n_available()` currently takes a
pointer to a `struct comedi_async`.  Change it to take a pointer to a
`struct comedi_subdevice` and use the `async` member for consistency
with the other comedi buffer handling functions.

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>