iommu/s390: Fix sparse warnings

Fix these warnings:
  CHECK   drivers/iommu/s390-iommu.c
drivers/iommu/s390-iommu.c:52:21: warning: symbol 's390_domain_alloc' was not declared. Should it be static?
drivers/iommu/s390-iommu.c:76:6: warning: symbol 's390_domain_free' was not declared. Should it be static?

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>