uorb: reduce RAM usage by avoiding string copies

The lifetime of the string is guaranteed because we never delete
DeviceNode objects and the strings in question are already on the heap.

This frees roughly 2.2KB of RAM on Pixracer

src/modules/uORB/uORBDevices.cpp

@@ -912,9 +912,9 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				} else {
 					// add to the node map;.
 #ifdef __PX4_NUTTX
-					_node_map.insert(nodepath, node);
+					_node_map.insert(devpath, node);
 #else
-					_node_map[std::string(nodepath)] = node;
+					_node_map[std::string(devpath)] = node;
 #endif
 				}