IB/mad cleanup: Clean up function params -- find_mad_agent

find_mad_agent only needs read only access to the MAD header.  Update the
ib_mad pointer to be const ib_mad_hdr.  Adjust call tree.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>