[media] Fix wrong register mask in gspca/sonixj.c

Hello,

When migrating from Slackware 13.1 to 13.37 (kernel 2.6.33.x to
2.6.37.6), there was some sort of regression with the external webcam
installed at the notebook (0x45:6128, SN9C325+OM6802).

In the version 2.6.37.6, the images got *very* dark, making the webcam
almost unusable, unless if used with direct sunlight.

Tracing back what happened, I concluded that changeset 0e4d413a
caused some sort of odd effects - including this - to this specific model.

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