(I think!) make the AR8327 switch correctly handle traffic.

This patch does four things:

* it globally disables mirroring;
* it globally sets the mirroring on each port to be disabled;
* the initial port setup now programs a portmask for the port to allow
  transmission (forwarding) to all other ports bar itself;
* the vlan setup path now programs the portmask for the port to
  allow transmission (forwarding) to all other ports bar itself.

Before this, I hard-coded the portmask to 0x3f which would mean all
ports (bar port 6, which currently isn't hooked up to anything.)
This means that traffic would be duplicated back out the port it
received it.  I bet this wasn't .. optimal.

In any case, this _seems_ to make DHCP from my macosx laptop
work through this access point.  I'll do some further testing
to ensure it's actually working correctly on all my devices.

Tested:

* DB120, AR8327 switch