V4L/DVB (7537): cx88/saa7134: Fix: avoid OOPS on module unload

If frontend is not attached, both cx88-dvb and saa7134-dvb don't
register DVB. However, dvb unregister were inconditionally called.

Due to that, an OOPS is generated.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
diff --git a/drivers/media/video/saa7134/saa7134-dvb.c b/drivers/media/video/saa7134/saa7134-dvb.c
index 5c84f45..e5c3569 100644
--- a/drivers/media/video/saa7134/saa7134-dvb.c
+++ b/drivers/media/video/saa7134/saa7134-dvb.c
@@ -1302,7 +1302,8 @@
 			}
 		}
 	}
-	videobuf_dvb_unregister(&dev->dvb);
+	if (dev->dvb.frontend)
+		videobuf_dvb_unregister(&dev->dvb);
 	return 0;
 }