V4L/DVB (13151): uvcvideo: Dismiss privileges when freeing video buffers

Dismiss privileges on the file handle when calling VIDIOC_REQBUFS with a
buffer count of 0. This allows applications to release the streaming
permissions on the file handle without closing it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>