x86/iommu: Use NULL instead of plain 0 for __IOMMU_INIT

IOMMU_INIT_POST and IOMMU_INIT_POST_FINISH pass the plain value
0 instead of NULL to __IOMMU_INIT. Fix this and make sparse
happy by doing so.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Joerg Roedel <joerg.roedel@amd.com>
Link: http://lkml.kernel.org/r/1346621506-30857-8-git-send-email-minipli@googlemail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>