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

Replace the global "vlan_tagged" variable with an array storing the
tagging state per vlan.

The code was taken from #12181, tested and cleaned up by Saverio Proto
with additional bug fixes supplied by Álvaro Fernández.

Tested-by: Jo-Philipp Wich <jow@openwrt.org>
Signed-off-by: Saverio Proto <zioproto@gmail.com>
Signed-off-by: Álvaro Fernández <noltari@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40777 3c298f89-4303-0410-b956-a3cf2f4a3e73