Revert "ar71xx: Fix tagged+untagged operation on AR8327N (#12181)"

Revert the tagged + untagged rework for now due to regressions in
vlan setup on certain AR83xx switches.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40842 3c298f89-4303-0410-b956-a3cf2f4a3e73