linux/dma-mapping.h: rename macro to prevent multiple inclusion

Having the macro to prevent multiple inclusion of
include/linux/dma-mapping.h contain the prefix "_ASM" is just begging
for possible confusion some day.
Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>

include/linux/dma-mapping.h

-#ifndef _ASM_LINUX_DMA_MAPPING_H
-#define _ASM_LINUX_DMA_MAPPING_H
+#ifndef _LINUX_DMA_MAPPING_H
+#define _LINUX_DMA_MAPPING_H
 
 #include <linux/device.h>
 #include <linux/err.h>