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

Backport or r35227.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@35228 3c298f89-4303-0410-b956-a3cf2f4a3e73