net: dsa: felix: stop migrating FDBs back and forth on tag proto change

I just realized we don't need to migrate the host-filtered FDB entries
when the tagging protocol changes from "ocelot" to "ocelot-8021q".

Host-filtered addresses are learned towards the PGID_CPU "multicast"
port group, reserved by software, which contains BIT(ocelot->num_phys_ports).
That is the "special" port entry in the analyzer block for the CPU port
module.

In "ocelot" mode, the CPU port module's packets are redirected to the
NPI port.

In "ocelot-8021q" mode, felix_8021q_cpu_port_init() does something funny
anyway, and changes PGID_CPU to stop pointing at the CPU port module and
start pointing at the physical port where the DSA master is attached.

The fact that we can alter the destination of packets learned towards
PGID_CPU without altering the MAC table entries themselves means that it
is pointless to walk through the FDB entries, forget that they were
learned towards PGID_CPU, and re-learn them towards the "unicast" PGID
associated with the physical port connected to the DSA master. We can
let the PGID_CPU value change simply alter the destination of the
host-filtered unicast packets in one fell swoop.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>