[media] dvb: Avoid warnings when compiled without the media controller

drivers/media/usb/dvb-usb-v2/dvb_usb_core.c: In function ‘dvb_usbv2_adapter_dvb_exit’:
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:531:25: warning: unused variable ‘d’ [-Wunused-variable]
  struct dvb_usb_device *d = adap_to_d(adap);
                         ^
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:403:13: warning: ‘dvb_usbv2_media_device_register’ defined but not used [-Wunused-function]
 static void dvb_usbv2_media_device_register(struct dvb_usb_adapter *adap)

drivers/media/usb/dvb-usb/dvb-usb-dvb.c:97:13: warning: ‘dvb_usb_media_device_register’ defined but not used [-Wunused-function]
 static void dvb_usb_media_device_register(struct dvb_usb_adapter *adap)
             ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>