Plane: unlock roll and pitch when we enter ACRO mode

ArduPlane/system.pde

@@ -340,10 +340,14 @@ static void set_mode(enum FlightMode mode)
     case MANUAL:
     case STABILIZE:
     case TRAINING:
-    case ACRO:
     case FLY_BY_WIRE_A:
         break;
 
+    case ACRO:
+        acro_state.locked_roll = false;
+        acro_state.locked_pitch = false;
+        break;
+
     case FLY_BY_WIRE_B:
         target_altitude_cm = current_loc.alt;
         break;