iommu: Fix return code in iommu_group_alloc_default_domain()

This function returns NULL on errors, not ERR_PTR.

Fixes: 1c68cbc ("iommu: Add IOMMU_DOMAIN_PLATFORM")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/8fb75157-6c81-4a9c-9992-d73d49902fa8@moroto.mountain
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/0-v2-ee2bae9af0f2+96-iommu_ga_err_ptr_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -1787,7 +1787,7 @@ iommu_group_alloc_default_domain(struct iommu_group *group, int req_type)
 	 */
 	if (ops->default_domain) {
 		if (req_type)
-			return ERR_PTR(-EINVAL);
+			return NULL;
 		return ops->default_domain;
 	}