staging: comedi: dt282x: fix IRQ assignment for dev->irq.

Here, dev->irq is not assigned with irq. comedi_legacy_detach()
is using dev->irq for release irq and dt282x_attach() is using dev->irq
for initialize comedi_subdevice.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/dt282x.c

@@ -1062,6 +1062,8 @@ static void dt282x_alloc_dma(struct comedi_device *dev,
 					   PAGE_SIZE, 0);
 	if (!devpriv->dma)
 		free_irq(irq_num, dev);
+	else
+		dev->irq = irq_num;
 }
 
 static void dt282x_free_dma(struct comedi_device *dev)