[Util] always convert liveness threshold to int

src/finn/util/fpgadataflow.py

@@ -51,4 +51,4 @@ def pyverilate_get_liveness_threshold_cycles():
     """Return the number of no-output cycles rtlsim will wait before assuming
     the simulation is not finishing and throwing an exception."""
 
-    return os.getenv("LIVENESS_THRESHOLD", 100)
+    return int(os.getenv("LIVENESS_THRESHOLD", 100))