staging: comedi: icp_multi: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the element type of the `ao_data[]` member of `struct
icp_multi_private` from `short` to `unsigned short` for consistency.

Also remove the `ai_data` and `di_data` members as they are not used.

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