generic: rtl8366_smi: only assign pvid if no pvid was set yet

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