cpu/esp8266: don't disconnect on pbuf full in send

Disconnecting from the AP in the send function if the lwIP packet buffer is exhausted is counterproductive since reconnecting usually fails on heavy network load. A better strategy is to slow down the sending of MAC frames from netif a bit to wait for flushing the buffer in the MAC layer.

cpu/esp8266/esp-wifi/esp_wifi_netdev.c

@@ -350,13 +350,9 @@ static int IRAM _send(netdev_t *netdev, const iolist_t *iolist)
         /*
          * The memory of EPS8266 is quite small. Therefore, it may happen on
          * heavy network load that we run into out of memory and we have
-         * to wait until lwIP pbuf has been flushed. For that purpose, we
-         * have to disconnect from AP and slow down sending. The node will
-         * then reconnect to AP automatically.
+         * to wait until lwIP pbuf has been flushed. We slow down sending a bit.
          */
         critical_exit();
-        /* disconnect from AP */
-        wifi_station_disconnect();
         /* wait 20 ms */
         xtimer_usleep(20 * US_PER_MS);