fix(dronecan): send elapsed ms since last seen in MSP2_INAV_DRONECAN_NODES

Previously last_seen_ms was the raw millis() timestamp when the node was
last heard from, which equals FC uptime for active nodes. Configurators
had no way to compute elapsed time without knowing current FC millis().

Now sends millis() - last_seen_ms so the field means "ms since this node
was last heard from". Unsigned subtraction handles millis() wraparound
correctly.

Author: daijoubu

src/main/fc/fc_msp.c

@@ -1899,7 +1899,7 @@ static bool mspFcProcessOutCommand(uint16_t cmdMSP, sbuf_t *dst, mspPostProcessF
                     .nodeID      = node->nodeID,
                     .health      = node->health,
                     .mode        = node->mode,
-                    .last_seen_ms = node->last_seen_ms,
+                    .last_seen_ms = millis() - node->last_seen_ms,
                 }, sizeof(dronecanNodeStatus_t));
             }
         }