V4L/DVB (9691): gspca: Move the video device to a separate area.

The video device was part of the gspca device. On device disconnection
while streaming, the device structure is freed at close time.
In this case, the remaining close job on the video device run out of
allocated memory.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>