dmaengine: at_hdmac: add a few const qualifiers

This prepares of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/dma/at_hdmac.c: In function 'at_dma_get_driver_data':
	drivers/dma/at_hdmac.c:1228: warning: return discards qualifiers from pointer target type

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

drivers/dma/at_hdmac.c

@@ -1216,7 +1216,7 @@ static const struct platform_device_id atdma_devtypes[] = {
 	}
 };
 
-static inline struct at_dma_platform_data * __init at_dma_get_driver_data(
+static inline const struct at_dma_platform_data * __init at_dma_get_driver_data(
 						struct platform_device *pdev)
 {
 	if (pdev->dev.of_node) {
@@ -1254,7 +1254,7 @@ static int __init at_dma_probe(struct platform_device *pdev)
 	int			irq;
 	int			err;
 	int			i;
-	struct at_dma_platform_data *plat_dat;
+	const struct at_dma_platform_data *plat_dat;
 
 	/* setup platform data for each SoC */
 	dma_cap_set(DMA_MEMCPY, at91sam9rl_config.cap_mask);