Update serial.c

"remove  #ifdef RT_SERIAL_USING_DMA in rt_serial_open， always check device open dma flag"

Author: chenyingchun0312

components/drivers/serial/serial.c

@@ -566,13 +566,11 @@ static rt_err_t rt_serial_open(struct rt_device *dev, rt_uint16_t oflag)
 
     LOG_D("open serial device: 0x%08x with open flag: 0x%04x",
         dev, oflag);
-#ifdef RT_SERIAL_USING_DMA
     /* check device flag with the open flag */
     if ((oflag & RT_DEVICE_FLAG_DMA_RX) && !(dev->flag & RT_DEVICE_FLAG_DMA_RX))
         return -RT_EIO;
     if ((oflag & RT_DEVICE_FLAG_DMA_TX) && !(dev->flag & RT_DEVICE_FLAG_DMA_TX))
         return -RT_EIO;
-#endif /* RT_SERIAL_USING_DMA */
     if ((oflag & RT_DEVICE_FLAG_INT_RX) && !(dev->flag & RT_DEVICE_FLAG_INT_RX))
         return -RT_EIO;
     if ((oflag & RT_DEVICE_FLAG_INT_TX) && !(dev->flag & RT_DEVICE_FLAG_INT_TX))