IB/ehca: Filter PATH_MIG events if QP was never armed

Certain firmware versions sometimes cause spurious PATH_MIG events to
occur during QP creation.  Filter these events by making sure PATH_MIG
events are only handed down when they actually make sense (i.e. when
the QP has been armed at least once).

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>