IB/mad: use CQ abstraction

Remove the local workqueue to process mad completions and use the CQ API
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hal Rosenstock <hal@mellanox.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>