staging: comedi: cb_pcidas64: sample types are unsigned

Sample values in comedi are unsigned.  Change the element type of
`ao_bounce_buffer[]` in `struct pcidas64_private` from `short` to
`unsigned short` for consistency.

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