net: dsa: b53: Do not program CPU port's PVID

The CPU port is special and does not need to obey VLAN restrictions as
far as untagged traffic goes, also, having the CPU port be part of a
particular PVID is against the idea of keeping it tagged in all VLANs.

Fixes: ca893194 ("net: dsa: b53: Keep CPU port as tagged in all VLANs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>