[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>