V4L/DVB (8812): gspca: Do pac73xx webcams work.

- documentation for some registers
- some preparations for adding autogain_n_exposure functionality
- various pac7311 fixes
- disable brightness and colors controls for 7311
- fix contrast control for 7311
- add hflip and vflip controls for 7311
- minimal jpeg header
- proper SOF detection

Signed-off-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>