ar71xx: ar8216: tagged+untagged on ar8327 (#12181)

This allows tagged and untagged traffic together on the same port on ar8327
switch devices.

I looked at the first attempt to do this in r40777 (ar71xx: Fix tagged+untagged
operation on AR8327N (#12181)). I also set the vlan and port egress policies
like that change. But I change vlan_tagged in an less intrusive way. The
tagged/untagged decision is now based on the following rules:
- if vid != pvid then traffic is always tagged
- if vid == pvid then vlan_tagged stores if the traffic should be tagged

Tested on TP-Link WDR-3600 (ar8327N).

Signed-off-by: Valentin Spreckels <Valentin.Spreckels@Informatik.Uni-Oldenburg.DE>

SVN-Revision: 42653