staging: comedi: pcmuio: pointers should be cleared with NULL

Pointer variables should be cleared with NULL on 0. This quiets
some sparse warnings about:

warning: Using plain integer as NULL pointer
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/pcmuio.c

@@ -442,7 +442,7 @@ static void pcmuio_stop_intr(struct comedi_device *dev,
 
 	subpriv->intr.enabled_mask = 0;
 	subpriv->intr.active = 0;
-	s->async->inttrig = 0;
+	s->async->inttrig = NULL;
 	nports = subpriv->intr.num_asic_chans / CHANS_PER_PORT;
 	firstport = subpriv->intr.asic_chan / CHANS_PER_PORT;
 	switch_page(dev, asic, PAGE_ENAB);
@@ -683,7 +683,7 @@ pcmuio_inttrig_start_intr(struct comedi_device *dev, struct comedi_subdevice *s,
 		return -EINVAL;
 
 	spin_lock_irqsave(&subpriv->intr.spinlock, flags);
-	s->async->inttrig = 0;
+	s->async->inttrig = NULL;
 	if (subpriv->intr.active)
 		event = pcmuio_start_intr(dev, s);