update: recurrent function cannot schedule another recurrent function per esp8266#6235 example

Author: d-a-v

cores/esp8266/Schedule.cpp

@@ -98,12 +98,9 @@ bool schedule_recurrent_function_us (const std::function<bool(void)>& fn, uint32
     item->mFunc = fn;
 
     if (rFirst)
-    {
         item->mNext = rFirst;
-        rFirst = item;
-    }
-    else
-        rFirst = item;
+
+    rFirst = item;
 
     return true;
 }

cores/esp8266/Schedule.h

@@ -58,7 +58,8 @@ void run_scheduled_functions();
 //   functions.  However a user function returning false will cancel itself.
 // * Long running operations or yield() or delay() are not allowed in the
 //   recurrent function.
-// * A recurrent scheduled function can schedule recurrent functions (if ever needed).
+// * A recurrent function currently must not schedule another recurrent
+//   functions.
 
 bool schedule_recurrent_function_us (const std::function<bool(void)>& fn, uint32_t repeat_us);