ar8216: adjust ATU flushing in case of link changes

If a link goes down, don't flush the complete ARL table.
Only flush the entries for the respective port.
Don't touch ARL table if a link goes up.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46381 3c298f89-4303-0410-b956-a3cf2f4a3e73