video: Add a note about the broken implementation

The cls command is broken. Previous discussion about this was at [1] and
[2]. For now, add a note to the source code.

[1] https://patchwork.ozlabs.org/project/uboot/patch/
       20221022092058.106052-1-heinrich.schuchardt@canonical.com/
[2] https://patchwork.ozlabs.org/project/uboot/patch/


       20230106145243.411626-12-sjg@chromium.org/

Signed-off-by: Simon Glass <sjg@chromium.org>