staging: comedi: adl_pci9118: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the `ao_data[]` and `dmabuf_virt[]` members of `struct
pci9118_private` and various local variables dealing with sample values
to use `unsigned short` instead of `short` for consistency.

Also remove the `short *ai_data` member of `struct pci9118_private` as
it is only assigned to.

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