staging: comedi: cb_pcidas: sample types are unsigned

Sample values in comedi are generally represented by unsigned values.
Change the element types of `ai_buffer[]`, `ao_buffer[]` and
`ao_value[]` in `struct cb_pcidas_private` to `unsigned short` for
consistency.

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