dmaengine: dw: update the driver comments

driver comment refers to DMA_SLAVE_CONFIG which needs to be updated to
.device_config
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/dw/regs.h

@@ -252,7 +252,7 @@ struct dw_dma_chan {
 	u8			src_master;
 	u8			dst_master;
 
-	/* configuration passed via DMA_SLAVE_CONFIG */
+	/* configuration passed via .device_config */
 	struct dma_slave_config dma_sconfig;
 };