staging: comedi: das16: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the type of the `data` variable in `das16_ai_munge()` 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>