staging: comedi: me4000: fix aref test in me4000_ai_check_chanlist()

The 'aref' (CR_AREF) values are all AREF_* defines. The SDF_* defines are
subdevice flags. Fix the test in this function.

Reported by: coverity
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/me4000.c

@@ -617,7 +617,7 @@ static int me4000_ai_check_chanlist(struct comedi_device *dev,
 			return -EINVAL;
 		}
 
-		if (aref == SDF_DIFF) {
+		if (aref == AREF_DIFF) {
 			if (chan >= max_diff_chan) {
 				dev_dbg(dev->class_dev,
 					"Channel number to high\n");