[ARM] Fix IOP13xx build warnings

http://armlinux.simtec.co.uk/kautobuild/2.6.27-rc5/iop13xx_defconfig/zimage.log

Occurrences  Warning text
339  arch/arm/include/asm/dma-mapping.h:40: warning: return makes pointer from integer without a cast
203  arch/arm/include/asm/dma-mapping.h:45: warning: return makes integer from pointer without a cast

Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>