stm32device: fix trace, add mask on byte value

Author: Guillaume Revaillot

src/main/java/org/stm32flash/STM32Device.java

@@ -540,7 +540,7 @@ private boolean readAck(int timeout) throws IOException, TimeoutException {
                 return false;
 
             default:
-                System.err.println("readAck: err, got unexpected 0x" + Integer.toHexString(b));
+                System.err.println("readAck: err, got unexpected 0x" + Integer.toHexString(b & 0xff));
                 return false;
         }
     }