mavlink_messages.cpp: only warn once, when MAVLink log opening fails

This happens for example if no SD card is present.

src/modules/mavlink/mavlink_messages.cpp

@@ -459,6 +459,7 @@ protected:
 #endif
 						} else {
 							PX4_WARN("Failed to open MAVLink log: %s", log_file_path);
+							_write_err_count = write_err_threshold; //only try to open the file once
 						}
 					}
 				}