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>

drivers/iommu/s390-iommu.c

@@ -49,7 +49,7 @@ static bool s390_iommu_capable(enum iommu_cap cap)
 	}
 }
 
-struct iommu_domain *s390_domain_alloc(unsigned domain_type)
+static struct iommu_domain *s390_domain_alloc(unsigned domain_type)
 {
 	struct s390_domain *s390_domain;
 
@@ -73,7 +73,7 @@ struct iommu_domain *s390_domain_alloc(unsigned domain_type)
 	return &s390_domain->domain;
 }
 
-void s390_domain_free(struct iommu_domain *domain)
+static void s390_domain_free(struct iommu_domain *domain)
 {
 	struct s390_domain *s390_domain = to_s390_domain(domain);