[bsp][Vango V85xx] modify NULL to RT_NULL

bsp/Vango_V85xx/drivers/drv_spi.c

@@ -127,7 +127,7 @@ static rt_uint32_t xfer(struct rt_spi_device* device, struct rt_spi_message* mes
     struct rt_spi_configuration * config = &device->config;
 
     RT_ASSERT(device != RT_NULL);
-    RT_ASSERT(message != NULL);
+    RT_ASSERT(message != RT_NULL);
 
     /* take CS */
     if(message->cs_take)