[media] em28xx: move video_device structs from struct em28xx to struct v4l2

There are 3 struct video_device at the em28xx common struct,
for video, VBI and radio. They all are used only by the V4L2 driver.
So, move them to the v4l2-specific struct.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>