net: dsa: mt7530: enable assisted learning on CPU port

Consider the following bridge configuration, where bond0 is not
offloaded:

         +-- br0 --+
        / /   |     \
       / /    |      \
      /  |    |     bond0
     /   |    |     /   \
   swp0 swp1 swp2 swp3 swp4
     .        .       .
     .        .       .
     A        B       C

Address learning is enabled on offloaded ports (swp0~2) and the CPU
port, so when client A sends a packet to C, the following will happen:

1. The switch learns that client A can be reached at swp0.
2. The switch probably already knows that client C can be reached at the
   CPU port, so it forwards the packet to the CPU.
3. The bridge core knows client C can be reached at bond0, so it
   forwards the packet back to the switch.
4. The switch learns that client A can be reached at the CPU port.
5. The switch forwards the packet to either swp3 or swp4, according to
   the packet's tag.

That makes client A's MAC address flap between swp0 and the CPU port. If
client B sends a packet to A, it is possible that the packet is
forwarded to the CPU. With offload_fwd_mark = 1, the bridge core won't
forward it back to the switch, resulting in packet loss.

As we have the assisted_learning_on_cpu_port in DSA core now, enable
that and disable hardware learning on the CPU port.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Vladimir Oltean <oltean@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>