iommu: Fix return code in iommu_group_alloc_default_domain()

This function returns NULL on errors, not ERR_PTR.

Fixes: 1c68cbc6 ("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>