ar8216: fix ATU flushing

ar8216: fix ATU flushing

The functionality to flush the address translation table contains two bugs
which luckily compensate each other.
1. Just setting the operation is not sufficient to perform the flushing.
   The "active" bit needs to be set to actually trigger an action.
   For the vtu operations this is implemented correctly.
2. ar8xxx_phy_read_status is called every 2s by the phy state machine
   to check for link changes. This would have caused an ATU flush
   every 2s.

Fix the chip-specific ATU flush functions and remove the ATU flush call
from ar8xxx_phy_read_status.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44101 3c298f89-4303-0410-b956-a3cf2f4a3e73