[arswitch] Fix ATU flushing on AR8216/AR8316 and most of the later chips.

The switch hardware requires this bit to be set in order to kick start the
actual ATU update. This was being masked on some chips by the learning
programming (what to do when a MAC address moves, hash table collision, etc)
which is currently inconsistent between chips.

Tested:

• AR9344 SoC (AR7240 style switch internal)