AP_RangeFinder: use have_serial when detecting

avoid find_serial() as it changes port options

libraries/AP_RangeFinder/AP_RangeFinder_Backend_Serial.cpp

@@ -53,7 +53,7 @@ uint32_t AP_RangeFinder_Backend_Serial::initial_baudrate(const uint8_t serial_in
 */
 bool AP_RangeFinder_Backend_Serial::detect(uint8_t serial_instance)
 {
-    return AP::serialmanager().find_serial(AP_SerialManager::SerialProtocol_Rangefinder, serial_instance) != nullptr;
+    return AP::serialmanager().have_serial(AP_SerialManager::SerialProtocol_Rangefinder, serial_instance);
 }