[media] em28xx: move some video-specific functions to em28xx-video

Now that we want to split the video handling to a separate
module, move all video-specific functions to em28xx-video.

No functional changes.

Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>