Make "controller nl:0" work again, by fixing a layering violation.

Make "controller nl:0" work again, by fixing a layering violation.

The kernel datapath was claiming that it implements STP, which is untrue.
In fact, the secchan implements STP, so if the secchan was not running
the STP support did not work.  Thus, "controller nl:0" would do the
wrong thing.

This makes the secchan edit the responses from the datapath to claim
STP support.