staging: comedi: pass subdevice to comedi_buf_get()

staging: comedi: pass subdevice to comedi_buf_get()

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

The function gets a sample value from the comedi buffer, but currently
only deals with 16-bit sample types.  A future version could deal with
16 or 32-bit sample types depending on the value of the SDF_LSAMPL
subdevice flag.

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>