staging: comedi: pcl812: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change `transfer_from_dma_buf()` and `interrupt_pcl812_ai_dma()` in the
"pcl812" module to use `unsigned short` sample values for consistency.

Also remove the `short *ai_data` member of `struct pcl812_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>