radv: Provide a toggle to avoid warnings about unsupported devices.

radv: Provide a toggle to avoid warnings about unsupported devices.

In the CI, we have such devices, and this message is printed many
hundreds of times. This results in a useless spam which makes it
difficult to see real issues.

v2 from Martin:
 - Reword the commit message a bit
 - Add a SoB

Signed-off-by: Martin Peres <martin.peres@mupuf.org>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Acked-by: Andres Gomez <agomez@igalia.com>
Acked-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11491>