[media] dvb-frontend: remove a warning

if CONFIG_MEDIA_CONTROLLER_DVB is not selected, it is now
producing this warning:

drivers/media/dvb-core/dvb_frontend.c: In function ‘dvb_frontend_thread’:
drivers/media/dvb-core/dvb_frontend.c:695:6: warning: unused variable ‘ret’ [-Wunused-variable]
  int ret;
      ^
Reported-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-core/dvb_frontend.c

@@ -692,7 +692,9 @@ static int dvb_frontend_thread(void *data)
 	struct dvb_frontend_private *fepriv = fe->frontend_priv;
 	fe_status_t s;
 	enum dvbfe_algo algo;
+#ifdef CONFIG_MEDIA_CONTROLLER_DVB
 	int ret;
+#endif
 
 	bool re_tune = false;
 	bool semheld = false;