[media] gspca - sn9c20x: Don't do sensor update before the capture is started

Telling the bridge to update the sensor when setting the exposure or the gain
is not needed when the image transfer is not started.

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