Instead of having lots of #ifdefs inside au0828-core due to
V4L2, move the dependencies to au0828-video.c. That allows
removing all those ifdefs, as au0828-video is only compiled if
CONFIG_VIDEO_AU0828_V4L2.

This fixes the following warnings reported by Kbuild test
with a random config with au0828 enabled, but V4L2 is disabled.

All warnings (new ones prefixed by >>):

   drivers/media/usb/au0828/au0828-core.c: In function 'au0828_usb_probe':
>> drivers/media/usb/au0828/au0828-core.c:463:1: warning: label 'done' defined but not used [-Wunused-label]
    done:
    ^
   drivers/media/usb/au0828/au0828-core.c: At top level:
   drivers/media/usb/au0828/au0828-core.c:250:12: warning: 'au0828_create_media_graph' defined but not used [-Wunused-function]
    static int au0828_create_media_graph(struct au0828_dev *dev)
               ^

Tested with a WinTV HVR 950Q (USB ID: 2040:7200)
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>