[media] gspca_main: wake wq on streamoff

We check for not streaming as a condition to abort waiting in dqbuf, so
when another thread does a streamoff we should wake the wq.

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