IB/mthca: reset QP's last pointers when transitioning to reset state

last pointer is not updated when QP is modified to reset state.  This
causes data corruption if WQEs are already posted on the queue.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>