V4L/DVB (8873): gspca: Bad image offset with rev012a of spca561 and adjust exposure.

-Make raw bayer header size change from 20 to 16 affect rev072a only, my 2
 rev012a cams both have a header size of 20
-While testing this I also tested the new exposure setting (good work on
 finding the register JF), and after quite a bit of testing have found out the
 exact meaning of the register, this patch modifies setexposure to control
 the exposure over a much wider range.

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>