staging: comedi: rtd520: use dev->read_subdev

Use the dev->read_subdev that was setup in the device attach instead
of accessing the dev->subdevices array directly.
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/rtd520.c

@@ -689,7 +689,7 @@ static int ai_read_dregs(struct comedi_device *dev, struct comedi_subdevice *s)
 static irqreturn_t rtd_interrupt(int irq, void *d)
 {
 	struct comedi_device *dev = d;
-	struct comedi_subdevice *s = &dev->subdevices[0];
+	struct comedi_subdevice *s = dev->read_subdev;
 	struct rtd_private *devpriv = dev->private;
 	u32 overrun;
 	u16 status;