net: dsa: install the primary unicast MAC address as standalone port host FDB

To be able to safely turn off CPU flooding for standalone ports, we need
to ensure that the dev_addr of each DSA slave interface is installed as
a standalone host FDB entry for compatible switches.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>