staging: comedi: use unsigned samples for comedi_buf_put()/get()

Sample values in comedi are generally represented as unsigned values.
`comedi_buf_put()` and `comedi_buf_get()` use a `short` to hold the
16-bit data value being transferred to or from the comedi buffer.
Change them to use `unsigned short` for consistency.

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