[media] gspca: sn9c2028: Add gain and autogain controls Genius Videocam Live v2

Autogain algorithm is very simple, if average luminance is low - increase gain,
if it's high - decrease gain. Gain granularity is low enough for this algo to
stabilize quickly.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>