iommu/amd: Return positive value in amd_iommu_detect()

Fix amd_iommu_detect() to return positive value on success, like
intended, and not zero. This will not change anything in the end
as AMD IOMMU disable swiotlb and properly associate itself with
devices even if detect() doesn't return a positive value.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: iommu@lists.linux-foundation.org