Calculate hops correctly even when hop_start==0 (#9120)

* Calculate hops correctly even when hop_start==0.

* Use the same type (int8_t) in the loop, avoiding signed/unsigned mismatches.

* Clarify defaultIfUnknown is returned for encrypted packets.

src/modules/RoutingModule.h

@@ -20,7 +20,7 @@ class RoutingModule : public ProtobufModule<meshtastic_Routing>
                                        uint8_t hopLimit = 0);
 
     // Given the hopStart and hopLimit upon reception of a request, return the hop limit to use for the response
-    uint8_t getHopLimitForResponse(uint8_t hopStart, uint8_t hopLimit);
+    uint8_t getHopLimitForResponse(const meshtastic_MeshPacket &mp);
 
   protected:
     friend class Router;