Disable retransmissions for internal flush uplinks

In certain cases, the modem can send an internal uplink message to flush
pending MAC commands that need to be sent to the network server. Disable
retransmissions when sending such uplink so that the values configured
by the application through AT+REP or AT+RTYNUM.

src/lrw.c

@@ -751,6 +751,16 @@ int lrw_send(uint8_t port, void *buffer, uint8_t length, bool confirmed)
         // Sertting the port to 0, the payload buffer to NULL, and buffer size
         // to 0 will send an uplink message with FOpts but no port or payload.
 
+        // Disable retransmissions for the internally generated flush uplink
+        // message.
+        r.Type = MIB_CHANNELS_NB_TRANS;
+        r.Param.ChannelsNbTrans = 1;
+        rc = LoRaMacMibSetRequestConfirm(&r);
+        if (rc != LORAMAC_STATUS_OK) {
+            log_debug("Could not configure retransmissions: %d", rc);
+            return rc;
+        }
+
         mr.Type = MCPS_UNCONFIRMED;
         mr.Req.Unconfirmed.fPort = 0;
         mr.Req.Unconfirmed.fBuffer = NULL;