[media] gspca - sonixj: Fix wrong register mask for sensor om6802

The bug was introduced by git commit 0e4d413a, giving very dark images.

Signed-off-by: Luiz Carlos Ramos <lramos.prof@yahoo.com.br>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>