V4L/DVB (8719): gspca: Have VIDIOC_QUERYCTRL more compliant to the spec.

- return -EINVAL when control not supported.
- start the private controls at V4L2_CID_PRIVATE_BASE.

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