cpu/esp8266: allow _esp_wifi_recv_cb during send

Receiption of a frame in _esp_wifi_recv_cb while sending has no effect and should be possible to increases the performance.

cpu/esp8266/esp-wifi/esp_wifi_netdev.c

@@ -98,7 +98,7 @@ static const struct station_config station_cfg = {
 
 #ifndef MODULE_ESP_NOW
 /**
- * Static const configuration for the SoftAP which is used to configure the 
+ * Static const configuration for the SoftAP which is used to configure the
  * SoftAP interface if ESP-NOW is not enabled.
  *
  * Since we need to use the WiFi interface in SoftAP + Station mode for
@@ -173,7 +173,7 @@ void IRAM _esp_wifi_recv_cb(struct pbuf *pb, struct netif *netif)
      * by a mutex because `esp_wifi_recv_cb` would be reentered from same
      * thread context.
      */
-    if (_in_esp_wifi_recv_cb || _in_send) {
+    if (_in_esp_wifi_recv_cb) {
         pbuf_free(pb);
         return;
     }

cpu/esp8266/esp-wifi/esp_wifi_netdev.h

@@ -20,7 +20,6 @@
 #define ESP_WIFI_NETDEV_H
 
 #include "net/netdev.h"
-#include "ringbuffer.h"
 
 #ifdef __cplusplus
 extern "C" {