hw/net: Declare link using static DEFINE_PROP_LINK() macro

Declare link statically using DEFINE_PROP_LINK().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20231017140150.44995-7-philmd@linaro.org>