V4L/DVB (9086): gspca: Use a kref to avoid potentialy blocking forever in disconnect.

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