[media] v4l: uvcvideo: Fix buffer completion size check

Commit e93e7fd9 ("v4l2: uvcvideo: Allow
using larger buffers") reworked the buffer size sanity check at buffer
completion time to use the frame size instead of the allocated buffer
size. However, it introduced two bugs in doing so:

- it assigned the allocated buffer size to the frame_size field, instead
  of assigning the correct frame size

- it performed the assignment in the S_FMT handler, resulting in the
  frame_size field being uninitialized if the userspace application
  doesn't call S_FMT.

Fix both issues by removing the frame_size field and validating the
buffer size against the UVC video control dwMaxFrameSize.

Fixes: e93e7fd9 ("v4l2: uvcvideo: Allow using larger buffers")

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