dma: ste_dma40: reuse is_slave_direction helper

The is_slave_direction helps to check if the transfer type is slave.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/ste_dma40.c

@@ -2337,7 +2337,7 @@ static struct dma_async_tx_descriptor *d40_prep_slave_sg(struct dma_chan *chan,
 							 unsigned long dma_flags,
 							 void *context)
 {
-	if (direction != DMA_DEV_TO_MEM && direction != DMA_MEM_TO_DEV)
+	if (!is_slave_direction(direction))
 		return NULL;
 
 	return d40_prep_sg(chan, sgl, sgl, sg_len, direction, dma_flags);