net: mscc: ocelot: move the logic to drop 802.1p traffic to the pvid deletion

Currently, the ocelot_port_set_native_vlan() function starts dropping
untagged and prio-tagged traffic when the native VLAN is removed?

What is the native VLAN? It is the only egress-untagged VLAN that ocelot
supports on a port. If the port is a trunk with 100 VLANs, one of those
VLANs can be transmitted as egress-untagged, and that's the native VLAN.

Is it wrong to drop untagged and prio-tagged traffic if there's no
native VLAN? Yes and no.

In this case, which is more typical, it's ok to apply that drop
configuration:
$ bridge vlan add dev swp0 vid 1 pvid untagged <- this is the native VLAN
$ bridge vlan add dev swp0 vid 100
$ bridge vlan add dev swp0 vid 101
$ bridge vlan del dev swp0 vid 1 <- delete the native VLAN
But only because the pvid and the native VLAN have the same ID.

In this case, it isn't:
$ bridge vlan add dev swp0 vid 1 pvid
$ bridge vlan add dev swp0 vid 100 untagged <- this is the native VLAN
$ bridge vlan del dev swp0 vid 101
$ bridge vlan del dev swp0 vid 100 <- delete the native VLAN

It's wrong, because the switch will drop untagged and prio-tagged
traffic now, despite having a valid pvid of 1.

The confusion seems to stem from the fact that the native VLAN is an
egress setting, while the PVID is an ingress setting. It would be
correct to drop untagged and prio-tagged traffic only if there was no
pvid on the port. So let's do just that.

Background:
https://lore.kernel.org/netdev/CA+h21hrRMrLH-RjBGhEJSTZd6_QPRSd3RkVRQF-wNKkrgKcRSA@mail.gmail.com/#t



Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>