ACPI/IORT: Fix build regression without IOMMU

A recent change reintroduced a bug that had previously been
fixed by commit d49f2ded ("ACPI/IORT: Fix CONFIG_IOMMU_API
dependency"):

drivers/acpi/arm64/iort.c: In function 'iort_iommu_configure':
drivers/acpi/arm64/iort.c:829:26: error: 'struct iommu_fwspec' has no member named 'ops'

Replace a direct reference to iommu_fwspec->ops with a helper function
call to fix the issue.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>