net: dsa: mv88e6xxx: Offload bridge broadcast flooding flag

These switches have two modes of classifying broadcast:

1. Broadcast is multicast.
2. Broadcast is its own unique thing that is always flooded
   everywhere.

This driver uses the first option, making sure to load the broadcast
address into all active databases. Because of this, we can support
per-port broadcast flooding by (1) making sure to only set the subset
of ports that have it enabled whenever joining a new bridge or VLAN,
and (2) by updating all active databases whenever the setting is
changed on a port.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>