staging: comedi: pcl818: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the interrupt data transfer functions in the "pcl818" module to
use unsigned types for consistency.

Also remove the `short *ai_data` member of `struct pcl818_private` as it
is only assigned to and otherwise unused.

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