net: dsa: Fix duplicate frames flooded by learning

When both the switch and the bridge are learning about new addresses,
switch ports attached to the bridge would see duplicate ARP frames
because both entities would attempt to send them.

Fixes: 5037d532 ("net: dsa: add Broadcom tag RX/TX handler")
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>