Rework vlan_add/vlan_del functions in preparation for dynamic cpu port.

Currently BIT(MT7530_CPU_PORT) is added to new_members, even though
mt7530_port_vlan_add() will be called on the CPU port too.

Let DSA core decide when to call port_vlan_add for the CPU port, rather
than doing it implicitly.

We can do autonomous forwarding in a certain VLAN, but not add br0 to that
VLAN and avoid flooding the CPU with those packets, if software knows it
doesn't need to process them.
Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>