IB/mad: Clean up comments in smi.c

Return values of 0 do not make sense for functions which return enum
smi_action

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