RDMA/cxgb4: Optimize CQ overflow detection

1) save the timestamp flit in the cq when we consume a CQE.

2) always compare the saved flit with the previous entry flit when
   reading the next CQE entry.  If the flits don't compare, then we
   have overflowed.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>