microbit-dal: Fixed a bug on printing a character with a delay

Previously when a blocking print finished, the animatemode was not
reset, which meant sometimes there would be a race condition.

source/MicroBitDisplay.cpp

@@ -232,8 +232,10 @@ MicroBitDisplay::animationUpdate()
             this->updateAnimateImage();
             
         if(animationMode == ANIMATION_MODE_PRINT_CHARACTER)
+        {
+            animationMode = ANIMATION_MODE_NONE;
             this->sendEvent(MICROBIT_DISPLAY_EVT_ANIMATION_COMPLETE);
-            
+        }
     }
 }