staging: comedi: pcl711: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Don't cast the sample value parameter of `comedi_buf_put()` to `short`,
particularly as it has now been changed to `unsigned short`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>