net: dsa: hellcreek: Add STP forwarding rule

Treat STP as management traffic. STP traffic is designated for the CPU port
only. In addition, STP traffic has to pass blocked ports.

Fixes: e4b27ebc ("net: dsa: Add DSA driver for Hirschmann Hellcreek switches")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>