staging: comedi: s626: pointers should be cleared with NULL not 0

pdma->LogicalBase is a void *, NULL should be used to clear it not 0.

This quiets a sparse warning about:

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

drivers/staging/comedi/drivers/s626.c

@@ -2779,7 +2779,7 @@ static void CloseDMAB(struct comedi_device *dev, struct bufferDMA *pdma,
 	vpptr = pdma->PhysicalBase;
 	if (vbptr) {
 		pci_free_consistent(devpriv->pdev, bsize, vbptr, vpptr);
-		pdma->LogicalBase = 0;
+		pdma->LogicalBase = NULL;
 		pdma->PhysicalBase = 0;
 
 		DEBUG("CloseDMAB(): Logical=%p, bsize=%d, Physical=0x%x\n",