V4L/DVB (11945): uvcvideo: Don't accept to change the format when buffers are allocated.

Setting a new frame format or size will likely change the buffer size required
to store a complete video frame. To avoid a buffer overflow, don't allow
VIDIOC_S_FMT calls when video buffers are already allocated.

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

diff --cc drivers/media/video/uvc/uvc_queue.c
Simple merge

diff --cc drivers/media/video/uvc/uvc_v4l2.c
Simple merge

diff --cc drivers/media/video/uvc/uvcvideo.h
Simple merge