net: dsa: felix: perform MDB migration based on ocelot->multicast list

The felix driver is the only user of dsa_port_walk_mdbs(), and there
isn't even a good reason for it, considering that the host MDB entries
are already saved by the ocelot switch lib in the ocelot->multicast list.

Rewrite the multicast entry migration procedure around the
ocelot->multicast list so we can delete dsa_port_walk_mdbs().

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