V4L/DVB (9686): gspca: Don't return the control values from the webcams in spca501.

The bridge takes some time to update after a write, so apps doing a quick
write/read can get the old value back.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>