Only assign a pvid if the current pvid is 0. Fixes using mixed tagged
and untagged traffic on a port and the untagged vlan isn't the last one
of which the port is a member.
Tested on RTL8366S and RTL8367R.
Fixes#8501.
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
SVN-Revision: 35227