IB/mthca: Fix SRQ cleanup during QP destroy

When cleaning up a CQ for a QP attached to SRQ, need to free an SRQ
WQE only if the CQE is a receive completion.

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>