rdtech-dps: rephrase how model specific ranges are handled

rdtech-dps: rephrase how model specific ranges are handled

Commit 02a4f485de76 implemented ranges support such that _all_ models
including every RD as well as DPS device announced ranges, it's just
that most of them happen to support a single range.

Use this fact, accept any zero based index, and drop the "normalization"
to strict 0/1 values except for one RD specific code path when getting
the current state of multi-range supporting devices. Tell models with
"effectively no ranges" from devices which "support multile ranges".
This shall increase robustness and improve maintainability. Simplifies
future extensions when more ranges are involved.