Move event packet handling into a serialized process context to remove

Move event packet handling into a serialized process context to remove
a state transitioning race condition.  Event packets, like completing
authentication and the following association completion, are usually
received shortly after another.  The code that handles those events is
scheduled using a task, so it can easily happen that the state change
caused by the authentication packet was not done before the following
association event arrived.  By moving the event packet handling into the
same context as the state task we serialize the processing and remove
the race condition.  Fixes connecting to the 5GHz WiFi AP used at a2k18.

ok stsp@