[media] gspca_main: Locking fixes 1

The gspca_dev->streaming boolean is protected against multiple access
through gspca_dev->queue_lock everywhere except for 2 places. This patch
fixes this by bringing it under the lock in vidioc_streamoff. And by
removing the check for gspca_dev->streaming in gspca_disconnect,
the destroy_urbs call may be called multiple times (and is protected
by the usb_lock) and calling wake_up_interruptible can also always be done
safely.

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>