V4L/DVB: gspca - zc3xx.c: Remove the brightness control

Brightness is done by the gamma tables and not by single bridge registers.
It will be back as soon as an algorithm will be found.

This change also fixes the autogain problem found by Jose Alberto Reguero
(webcam 046d:08dd).

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