[media] gspca - zc3xx: Fix bad sensor values when changing autogain

The setexposure added in commit 590f2168

 works only for the sensor hv7131r,
but it is called for all sensors when changing the autogain.

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