cpu/esp8266: dbg msg replaced by an error msg

The situation where the firmware `lwIP` packet buffer is exhausted is an important indication that the traffic sent to and sent from the esp8266 is more than the esp8266 is able to handle. Therefore, it should be an error message.

cpu/esp8266/esp-wifi/esp_wifi_netdev.c

@@ -394,7 +394,7 @@ static int IRAM _send(netdev_t *netdev, const iolist_t *iolist)
 
     struct pbuf *pb = pbuf_alloc(PBUF_LINK, iol_len + PBUF_IEEE80211_HLEN, PBUF_RAM);
     if (pb == NULL || pb->tot_len < iol_len) {
-        ESP_WIFI_DEBUG("could not allocate buffer to send %d bytes ", iol_len);
+        ESP_WIFI_LOG_ERROR("could not allocate buffer to send %d bytes ", iol_len);
         /*
          * 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