fix bug in RLC AM where poll bit was never set if poll_byte and poll_pdu are zero

- In case both poll_pdu and poll_byte are zero a status PDU is requested
  from the other AM entity every N data PDUs

lib/include/srslte/upper/rlc_am.h

@@ -163,6 +163,8 @@ private:
 
   static const int reordering_timeout_id = 1;
 
+  static const int poll_periodicity = 8; // After how many data PDUs a status PDU shall be requested
+
   // Timer checks
   bool status_prohibited();
   bool poll_retx();

lib/src/upper/rlc_am.cc

@@ -381,6 +381,18 @@ bool rlc_am::poll_required()
     return true;
   if(poll_retx())
     return true;
+
+  if(tx_sdu_queue.size() == 0 && retx_queue.size() == 0)
+    return true;
+
+  /* According to 5.2.2.1 in 36.322 v13.3.0 a poll should be requested if
+   * the entire AM window is unacknowledged, i.e. no new PDU can be transmitted.
+   * However, it seems more appropiate to request more often if polling
+   * is disabled otherwise, e.g. every N PDUs.
+   */
+  if (cfg.poll_pdu == 0 && cfg.poll_byte == 0 && vt_s % poll_periodicity == 0)
+    return true;
+
   return false;
 }