The driver was relying on dsa_slave_vlan_rx_add_vid to add VLAN ID 0. After
the blamed commit, VLAN ID 0 won't be set up anymore, breaking software
bridging fallback on VLAN-unaware bridges.

Manually set up VLAN ID 0 to fix this.

Fixes: 06cfb2df ("net: dsa: don't advertise 'rx-vlan-filter' when not needed")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>