[media] gspca - stv06xx: Set a lower default value of gain for hdcs sensors

Many users said that the default gain value (128) was giving white images.
The value which was in the original qc-usb driver (50) is better.

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