staging: comedi: dt3000: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the type of the `data` variable in `dt3k_ai_empty_fifo()` 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/dt3000.c

@@ -331,7 +331,7 @@ static void dt3k_ai_empty_fifo(struct comedi_device *dev,
 	int rear;
 	int count;
 	int i;
-	short data;
+	unsigned short data;
 
 	front = readw(devpriv->io_addr + DPR_AD_Buf_Front);
 	count = front - devpriv->ai_front;