staging: comedi: cb_pcidas64: sample types are unsigned

Sample values in comedi are unsigned.  Change the element type of
`ao_bounce_buffer[]` in `struct pcidas64_private` from `short` to
`unsigned short` for consistency.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/cb_pcidas64.c

@@ -1137,7 +1137,7 @@ struct pcidas64_private {
 	volatile short ai_cmd_running;
 	unsigned int ai_fifo_segment_length;
 	struct ext_clock_info ext_clock;
-	short ao_bounce_buffer[DAC_FIFO_SIZE];
+	unsigned short ao_bounce_buffer[DAC_FIFO_SIZE];
 };
 
 static unsigned int ai_range_bits_6xxx(const struct comedi_device *dev,