mtd: use NULL instead of 0 for an address

Use NULL instead of 0 when returning an address. This fixes a
sparse warning.
Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/maps/pcmciamtd.c

@@ -89,7 +89,7 @@ static caddr_t remap_window(struct map_info *map, unsigned long to)
 
 	if (!pcmcia_dev_present(dev->p_dev)) {
 		pr_debug("device removed\n");
-		return 0;
+		return NULL;
 	}
 
 	offset = to & ~(dev->win_size-1);