staging: comedi: me4000: remove 'board' from me4000_ai_check_chanlist()

The maximum differential channel is half the subdevice 'n_chan'. Use
that instead and remove the need for the 'board' variable.

Also, the comedi core does no validate the aref flags. Add a check
to ensure that the subdevice actually supports the AREF_DIFF mode.
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

@@ -568,8 +568,6 @@ static int me4000_ai_check_chanlist(struct comedi_device *dev,
 				    struct comedi_subdevice *s,
 				    struct comedi_cmd *cmd)
 {
-	const struct me4000_board *board = dev->board_ptr;
-	unsigned int max_diff_chan = board->ai_diff_nchan;
 	unsigned int aref0 = CR_AREF(cmd->chanlist[0]);
 	int i;
 
@@ -585,7 +583,13 @@ static int me4000_ai_check_chanlist(struct comedi_device *dev,
 		}
 
 		if (aref == AREF_DIFF) {
-			if (chan >= max_diff_chan) {
+			if (!(s->subdev_flags && SDF_DIFF)) {
+				dev_err(dev->class_dev,
+					"Differential inputs are not available\n");
+				return -EINVAL;
+			}
+
+			if (chan >= (s->n_chan / 2)) {
 				dev_dbg(dev->class_dev,
 					"Channel number to high\n");
 				return -EINVAL;