V4L/DVB (8350): gspca: Conform to v4l2 spec and mutex unlock fix.

gspca:    Empty the application queues on streamoff ioctl.
          streamon/off and qbuf ioctls may be done by any application.
          Don't handle MJPEG pixel format as JPEG.
                (thanks to Hans de Goede)
          Authorize reqbufs ioctl when streaming.
          Bad mutex unlock in poll() when streaming problem.
                (thanks to Mauro Carvalho Chehab)
          Don't handle 'webcam disconnected' in stream off.

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