V4L/DVB (8870): gspca: Fix dark room problem with sonixb.

When using the sonixb driver in a dark room and given that the autoexposure
algorithm starts with a setting most suitable for daylight, the picture
produced by the cam may actually be 100% black leading to a avg_lum value of 0,
so an avg_lum value of 0 does not always signal an exposure settings change
(which it normally does). This patch adds a check for the really black image
case and stops dropping all frames as invalid in this case.

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>