Purge the cached nodes when we switch back to SCAN. In case we lost

Purge the cached nodes when we switch back to SCAN.  In case we lost
the connection because an AP is gone away, this makes sure we don't
end up picking a now non-existant AP over and over.

Another problem this works around is that when trying to re-join that
AP, the attempt to connect would not yield any event message from the
firmware, and then we would get stuck.  Since we now definitely choose
a different AP, this issue does not show up.

ok stsp@