[io/Future] take a const reference of the Future::Callback

emper/io/Future.hpp

@@ -233,7 +233,7 @@ class Future : public Logger<LogSubsystem::IO> {
 	 * @param callback Callback reference which is copied and executed on completion.
 	 *                 It gets passed the value causing the completion.
 	 */
-	inline void setCallback(Callback& callback) {
+	inline void setCallback(const Callback& callback) {
 		/* Call copy constructor */
 		setCallback(new Callback(callback));
 	}
@@ -244,7 +244,7 @@ class Future : public Logger<LogSubsystem::IO> {
 	 * @param callback Callback rvalue which gets moved and executed on completion.
 	 *                 It gets passed the value causing the completion.
 	 */
-	inline void setCallback(Callback&& callback) {
+	inline void setCallback(const Callback&& callback) {
 		/* Call move constructor */
 		setCallback(new Callback(callback));
 	}