[media] gspca - sn9c20x: Add the JPEG compression quality control

The JPEG compression quality was hardcoded to 95%. This value was too big,
raising often buffer overflows.
This quality is now 80% by default and is settable.

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