IB/ipath: Implement IB_EVENT_QP_LAST_WQE_REACHED

This patch implements the IB_EVENT_QP_LAST_WQE_REACHED event which is
needed by ib_ipoib to destroy the QP when used in connected mode.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>