AMD IOMMU: use iommu_device_max_index, fix

include/linux/iommu-helper.h has no header guards, which breaks
sparc64 build. Add them.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Joerg Roedel <joerg.roedel@amd.com>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>