net: mscc: ocelot: allow a config where all bridge VLANs are egress-untagged

At present, the ocelot driver accepts a single egress-untagged bridge
VLAN, meaning that this sequence of operations:

ip link add br0 type bridge vlan_filtering 1
ip link set swp0 master br0
bridge vlan add dev swp0 vid 2 pvid untagged

fails because the bridge automatically installs VID 1 as a pvid & untagged
VLAN, and vid 2 would be the second untagged VLAN on this port. It is
necessary to delete VID 1 before proceeding to add VID 2.

This limitation comes from the fact that we operate the port tag, when
it has an egress-untagged VID, in the OCELOT_PORT_TAG_NATIVE mode.
The ocelot switches do not have full flexibility and can either have one
single VID as egress-untagged, or all of them.

There are use cases for having all VLANs as egress-untagged as well, and
this patch adds support for that.

The change rewrites ocelot_port_set_native_vlan() into a more generic
ocelot_port_manage_port_tag() function. Because the software bridge's
state, transmitted to us via switchdev, can become very complex, we
don't attempt to track all possible state transitions, but instead take
a more declarative approach and just make ocelot_port_manage_port_tag()
figure out which more to operate in:

- port is VLAN-unaware: the classified VLAN (internal, unrelated to the
                        802.1Q header) is not inserted into packets on egress
- port is VLAN-aware:
  - port has tagged VLANs:
    -> port has no untagged VLAN: set up as pure trunk
    -> port has one untagged VLAN: set up as trunk port + native VLAN
    -> port has more than one untagged VLAN: this is an invalid config
       which is rejected by ocelot_vlan_prepare
  - port has no tagged VLANs
    -> set up as pure egress-untagged port

We don't keep the number of tagged and untagged VLANs, we just count the
structures we keep.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>