Interestingly enough, the following does not cause an error although I think it is equivalent.
-mca rmaps_base_mapping_policy dist:span -mca rmaps_dist_device ${hca_dev}
As I understand it this should be the same as -map-by dist:${hca_dev},span ???
It is a bit hard to find out if the binding is correct because on the system I have available this mapping is the same as the natural mapping...