Update stm32_eth.c

Fix some exception handling，such as when rt_sem_take failed in tx，transmission cannot be resumed。

bsp/stm32f107/drivers/stm32_eth.c

@@ -3206,7 +3206,12 @@ rt_err_t rt_stm32_eth_tx( rt_device_t dev, struct pbuf* p)
     {
         rt_err_t result;
         result = rt_sem_take(&tx_buf_free, 2);
-        if (result != RT_EOK) return -RT_ERROR;
+        if (result != RT_EOK)
+        {
+            ETH_FlushTransmitFIFO();  // clear fifo
+            ETH_ResumeDMATransmission();	 //	resume dma	
+            return -RT_ERROR;
+        }
     }
 
     offset = 0;