[media] gspca_main: Locking fixes 2

Before this patch vidioc_dqbuf is using its own read_lock, where as
other queue related functions use queue_lock. This means that dqbuf is
accessing several variables in a racy manor. The most important one
being fr_o, which may be changed from underneath dqbuf by vidioc_reqbufs
or vidioc_streamoff. Other variables which it accesses unprotected
are gspca_dev->memory, gspca_dev->streaming and gspca_dev->capt_file.

This patch fixes this by changing vidioc_dqbuf to also use the queue_lock.

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>