[media] cx231xx: fix compilation if the media controller is not defined

[media] cx231xx: fix compilation if the media controller is not defined

drivers/media/usb/cx231xx/cx231xx-cards.c: In function ‘cx231xx_usb_probe’:
drivers/media/usb/cx231xx/cx231xx-cards.c:1589:15: error: ‘struct v4l2_device’ has no member named ‘mdev’
  dev->v4l2_dev.mdev = dev->media_dev;
               ^
drivers/media/usb/cx231xx/cx231xx-cards.c:1589:26: error: ‘struct cx231xx’ has no member named ‘media_dev’
  dev->v4l2_dev.mdev = dev->media_dev;
                          ^
scripts/Makefile.build:257: recipe for target 'drivers/media/usb/cx231xx/cx231xx-cards.o' failed

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>