[DeviceDriver][Serial] Fix serial open flag lost when device reopen.

components/drivers/serial/serial.c

@@ -641,6 +641,13 @@ static rt_err_t rt_serial_open(struct rt_device *dev, rt_uint16_t oflag)
             serial->serial_rx = RT_NULL;
         }
     }
+    else
+    {
+        if (oflag & RT_DEVICE_FLAG_DMA_RX)
+            dev->open_flag |= RT_DEVICE_FLAG_DMA_RX;
+        else if (oflag & RT_DEVICE_FLAG_INT_RX)
+            dev->open_flag |= RT_DEVICE_FLAG_INT_RX;
+    }
 
     if (serial->serial_tx == RT_NULL)
     {
@@ -676,6 +683,13 @@ static rt_err_t rt_serial_open(struct rt_device *dev, rt_uint16_t oflag)
             serial->serial_tx = RT_NULL;
         }
     }
+    else
+    {
+        if (oflag & RT_DEVICE_FLAG_DMA_TX)
+            dev->open_flag |= RT_DEVICE_FLAG_DMA_TX;
+        else if (oflag & RT_DEVICE_FLAG_INT_TX)
+            dev->open_flag |= RT_DEVICE_FLAG_INT_TX;
+    }
 
     /* set stream flag */
     dev->open_flag |= stream_flag;