Add ATU flush support.

The OpenWRT AR8xxx switch support flushes the ATU (address translation
unit) after each port link 'up' status change. I've modified this to
just flush on any port transition.

Whilst here, bump the number of ports on the AR8327 to 6, rather than
the default of 5. It's DB120 specific; I'll go and make this configurable
later.

There's some debugging code in here still; I am still debugging whether
this is or isn't working fully.

Tested:

• DB120, AR9344 + AR8327 switch

Obtained from: OpenWRT