V4L/DVB (9689): gspca: Memory leak when disconnect while streaming.

As a side effect, the sd routine stop0 is called on disconnect.
This permits the subdriver to free its resources.

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