intel-iommu: Fix 32-bit build warning with __cmpxchg()

drivers/pci/intel-iommu.c: In function 'dma_pte_addr':
drivers/pci/intel-iommu.c:239: warning: passing argument 1 of '__cmpxchg64' from incompatible pointer type

It seems that __cmpxchg64() now cares about the type of its pointer argument,
so give it a (uint64_t *) instead of a pointer to a structure which contains
only that.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>