net: dsa: mv88e6xxx: bridges do not need an FID

With 88E6352 and similar switch chips, each port has a map to restrict
which output port this input port can egress frames to.

The current driver code implements hardware bridging using this feature,
and assigns to a bridge group the FID of its first member.

Now that 802.1Q is fully implemented in this driver, a Linux bridge
which is a simple untagged VLAN, already gets its own FID.

This patch gets rid of the per-bridge FID and explicits the usage of the
port based VLAN map feature.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>