rlc_am_lte: refactor logging when PDCP SDUs are enqueued

the patch refactor the logging when a new PDCP SDU is enqueued for
transmission at RLC.

If the SN is already present, only a warning is logged. From the RLC
perspective operation continues and the SDU will be transmitted.

The patch also changes the order of logs. When the SN cannot be inserted
inside the queue of undelivered SDUs, only one message is logged.

lib/src/upper/rlc_am_lte.cc

@@ -439,15 +439,14 @@ int rlc_am_lte::rlc_am_lte_tx::write_sdu(unique_byte_buffer_t sdu)
     return SRSLTE_ERROR;
   }
 
-  // Store SDU info
-  logger.debug(
-      "Storing PDCP SDU info in queue. PDCP_SN=%d, Queue Size=%ld", sdu_pdcp_sn, undelivered_sdu_info_queue.nof_sdus());
-
   if (undelivered_sdu_info_queue.has_pdcp_sn(sdu_pdcp_sn)) {
-    logger.error("PDCP SDU info already exists. SN=%d", sdu_pdcp_sn);
+    logger.warning("PDCP_SN=%d already marked as undelivered", sdu_pdcp_sn);
     return SRSLTE_ERROR;
   }
 
+  // Store SDU info
+  logger.debug("Marking PDCP_SN=%d as undelivered (queue_len=%ld)", sdu_pdcp_sn, undelivered_sdu_info_queue.nof_sdus());
+
   undelivered_sdu_info_queue.add_pdcp_sdu(sdu_pdcp_sn);
   return SRSLTE_SUCCESS;
 }