IB/mthca: Simplify calls to mthca_cq_clean()

If a QP has separate send and receive CQs, then the send CQ will never
have receive completions from that QP in it.  So when cleaning the
send CQ, there's no need to pass in an SRQ pointer, even if the QP is
attached to an SRQ.

Signed-off-by: Roland Dreier <rolandd@cisco.com>