staging: comedi: adv_pci1710: 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
pci1710_private` from `short` to `unsigned short` for consistency.  Also
remove the `ai_data` member as it is only assigned to.  Change various
local variables used to hold sample values to `unsigned short`.

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