staging: comedi: dt282x: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change various members of `struct dt282x_private` and various parameters
and variables dealing with samples to use `unsigned short` instead of
`short` for consistency.

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