[bsp][ch32] fix the empty function ch32dma_transmit return error code as -RT_EIO

bsp/wch/risc-v/Libraries/ch32_drivers/drv_usart.c

@@ -345,7 +345,7 @@ static int ch32_getc(struct rt_serial_device *serial)
 
 static rt_ssize_t ch32dma_transmit(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
 {
-    return RT_EOK;
+    return -RT_EIO;
 }
 
 static void uart_isr(struct rt_serial_device *serial)