wireless/bcm43xxx: enable tx flow control to improve performance

RX/TX shared free queue on bcmf implementation, if TX occupies the
free queue completely, RX will trigger read abort because it cannot
alloc buffer successfully from the shared free queue. This commit will
limit the sending entries of tx and prevent rx triggering abort

Signed-off-by: chao.an <anchao@xiaomi.com>

drivers/wireless/ieee80211/bcm43xxx/bcmf_sdio.c

@@ -1065,11 +1065,9 @@ struct bcmf_sdio_frame *bcmf_sdio_allocate_frame(FAR struct bcmf_dev_s *priv,
           DEBUGPANIC();
         }
 
-#if 0
       if (!tx ||
           sbus->tx_queue_count <
-            CONFIG_IEEE80211_BROADCOM_FRAME_POOL_SIZE - 1)
-#endif
+            CONFIG_IEEE80211_BROADCOM_FRAME_POOL_SIZE / 2)
         {
           if ((entry = bcmf_dqueue_pop_tail(&sbus->free_queue)) != NULL)
             {