net: dsa: lan9303: Add basic offloading of unicast traffic

When both user ports are joined to the same bridge, the normal
HW MAC learning is enabled. This means that unicast traffic is forwarded
in HW.

If one of the user ports leave the bridge,
the ports goes back to the initial separated operation.

Port separation relies on disabled HW MAC learning. Hence the condition
that both ports must join same bridge.

Add brigde methods port_bridge_join, port_bridge_leave and
port_stp_state_set.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: David S. Miller <davem@davemloft.net>