media: v4l2-ctrls: return -EACCES if request wasn't completed

For now (this might be relaxed in the future) we do not allow getting
controls from a request that isn't completed. In that case we return
-EACCES. Update the documentation accordingly.

Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>