iommu: fix initialization without 'add_device' callback

IOMMU drivers can be initialized from of_iommu helpers. Such drivers don't
need to provide device_add callbacks to operate properly, so there is no
need to fail initialization if the callback is missing.
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

drivers/iommu/iommu.c

@@ -737,7 +737,7 @@ static int add_iommu_group(struct device *dev, void *data)
 	const struct iommu_ops *ops = cb->ops;
 
 	if (!ops->add_device)
-		return -ENODEV;
+		return 0;
 
 	WARN_ON(dev->iommu_group);