amd-iommu: detach device explicitly before attaching it to a new domain

This fixes a bug with a device that could not be assigned to a KVM guest
because it is still assigned to a dma_ops protection domain.

[chrisw: simply remove WARN_ON(), will always fire since dev->driver
will be pci-sub]
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

arch/x86/kernel/amd_iommu.c

@@ -2073,7 +2073,7 @@ static int amd_iommu_attach_device(struct iommu_domain *dom,
 
 	old_domain = domain_for_device(devid);
 	if (old_domain)
-		return -EBUSY;
+		detach_device(old_domain, devid);
 
 	attach_device(iommu, domain, devid);