[media] tw5864: constify struct video_device template

tw5864_video_template is used for filling of actual video_device
structures. It is copied by value, and is not used for anything else.

Signed-off-by: Andrey Utkin <andrey.utkin@corp.bluecherry.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>