device.h: dma_mem is only needed for HAVE_GENERIC_DMA_COHERENT

No need to carry an unused field around.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/device.h

@@ -1017,8 +1017,10 @@ struct device {
 
 	struct list_head	dma_pools;	/* dma pools (if dma'ble) */
 
+#ifdef CONFIG_HAVE_GENERIC_DMA_COHERENT
 	struct dma_coherent_mem	*dma_mem; /* internal for coherent mem
 					     override */
+#endif
 #ifdef CONFIG_DMA_CMA
 	struct cma *cma_area;		/* contiguous memory area for dma
 					   allocations */