Minimal fix for in-band control loop with hub.

Minimal fix for in-band control loop with hub.

When a hub has two switches and the controller on three of its ports,
control traffic from the controller to switch 1 is seen by switch 2 as
data traffic, and switch 2 sends it up over its OpenFlow connection.  That
control traffic is in turn seen by the switch 1 as data traffic, and so
switch 1 sends it up over its OpenFlow connection.  And so on: this is a
loop.

This commit fixes the situation by not sending up over the OpenFlow
connection traffic to the controller that arrived on the controller's
port, as well as traffic from the controller.