IB/mad: Clean up rcv_has_same_class

rcv_has_same_class only needs access to the MAD header
specify WR and Receive WC as const

Reviewed-By: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>