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>

drivers/media/video/cx88/cx88-dvb.c

@@ -951,6 +951,7 @@ static int cx8802_dvb_remove(struct cx8802_driver *drv)
 	struct cx8802_dev *dev = drv->core->dvbdev;
 
 	/* dvb */
+	if (dev->dvb.frontend)
 		videobuf_dvb_unregister(&dev->dvb);
 
 	vp3054_i2c_remove(dev);

drivers/media/video/saa7134/saa7134-dvb.c

@@ -1302,6 +1302,7 @@ static int dvb_fini(struct saa7134_dev *dev)
 			}
 		}
 	}
+	if (dev->dvb.frontend)
 		videobuf_dvb_unregister(&dev->dvb);
 	return 0;
 }