staging: comedi: pass subdevice to comedi_buf_read_n_available()

Change the parameters of `comedi_buf_read_n_available()` to pass a
pointer to the comedi subdevice instead of a pointer to the "async"
structure belonging to the subdevice.

The main aim at the moment is to replace all the `struct comedi_async *`
parameters with `struct comedi_subdevice *` parameters in the comedi
driver API.

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>