iw_cxgb4: add queue drain functions

Add completion objects, named sq_drained and rq_drained, to the c4iw_qp
struct.  The queue-specific completion object is signaled when the last
CQE is drained from the CQ for that queue.

Add c4iw_drain_sq() to block until qp->rq_drained is completed.

Add c4iw_drain_rq() to block until qp->sq_drained is completed.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>