[media] gspca: Switch to V4L2 core locking, except for the buffer que…

…uing ioctls

Due to latency concerns the VIDIOC_QBUF, DQBUF and QUERYBUF do not use the
core lock, instead they rely only on queue_lock.

Changes by HdG:
1) Change release from the video_device to the v4l2_device, to avoid a
race on disconnect.
2) Adjust for the V4L2 core changes which cause non ioctl fops to no longer
take the V4L2 core lock.

[mchehab@redhat.com: fix a merge conflict]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>