[media] em28xx-v4l: fix the freeing of the video devices memory

Remove some dead code from em28xx_v4l2_fini() and fix the leaking of the video,
vbi and radio video_device struct memories.

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