drivers/video/mx3fb.c: use NULL for pointer

Fix the following sparse error:

  drivers/video/mx3fb.c:1309:28: warning: Using plain integer as NULL pointer
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/mx3fb.c

@@ -1306,7 +1306,7 @@ static int mx3fb_unmap_video_memory(struct fb_info *fbi)
 	dma_free_writecombine(fbi->device, fbi->fix.smem_len,
 			      fbi->screen_base, fbi->fix.smem_start);
 
-	fbi->screen_base = 0;
+	fbi->screen_base = NULL;
 	mutex_lock(&fbi->mm_lock);
 	fbi->fix.smem_start = 0;
 	fbi->fix.smem_len = 0;