net: dsa: mv88e6xxx: override existent unicast portvec in port_fdb_add

Having multiple destination ports for a unicast address does not make
sense.
Make port_db_load_purge override existent unicast portvec instead of
adding a new port bit.

Fixes: 88472939

 ("net: dsa: mv88e6xxx: handle multiple ports in ATU")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20210130134334.10243-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>