add tusb_inited() API, check for inited() before running device/host task

src/tusb.c

@@ -68,6 +68,11 @@ bool tusb_init(void)
   return TUSB_ERROR_NONE;
 }
 
+bool tusb_inited(void)
+{
+  return _initialized;
+}
+
 /*------------------------------------------------------------------*/
 /* Debug
  *------------------------------------------------------------------*/