staging: comedi: fl512: sample types are unsigned

Sample values in comedi are generally represented as unsigned values.
Change the element type of the `ao_readback[]` member of `struct
fl512_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/fl512.c

@@ -25,8 +25,7 @@ Configuration options:
 
 #define FL512_SIZE 16		/* the size of the used memory */
 struct fl512_private {
-
-	short ao_readback[2];
+	unsigned short ao_readback[2];
 };
 
 static const struct comedi_lrange range_fl512 = { 4, {