[media] cx25821: fix compiler warning

drivers/media/pci/cx25821/cx25821-video.c: In function ‘cx25821_video_register’:
drivers/media/pci/cx25821/cx25821-video.c:518:1: warning: the frame size of 1600 bytes is larger than 1024 bytes [-Wframe-larger-than=]
Fixed by just making the struct video_device template static const.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>