V4L/DVB (8116): videodev: allow PRIVATE_BASE controls when called through VIDIOC_G/S_CTRL.

V4L2_CID_PRIVATE_BASE controls are not allowed when called from
VIDIOC_S/G_EXT_CTRL as extended controls use a better mechanism
for private controls. But still allow it when called from the
VIDIOC_G/S_CTRL to extended control conversion in video_ioctl2()
for backwards compatibility.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>