[media] gspca - sn9c20x: Add automatic JPEG compression mechanism

The JPEG compression may be adjusted from the packet fill ratio and from
the flag 'USB FIFO full' returned in each frame.
The code is adapted from the one in gspca sonixj and uses a workqueue.

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