net: dsa: lan9303: Learn addresses on CPU port when bridged

When CPU transmit directly to port using tag, the LAN9303 does not
learn MAC addresses received on the CPU port into the ALR.
ALR learning is performed only when transmitting using ALR lookup.

Solution:
If the two external ports are bridged and the packet is not STP BPDU,
then use ALR lookup to allow ALR learning on CPU port.
Otherwise transmit directly to port with STP state override.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>