iommu/amd: Do not BUG_ON in __detach_device()

The condition in the BUG_ON is an indicator of a BUG, but no
reason to kill the code path. Turn it into a WARN_ON and
bail out if it is hit.
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd_iommu.c

@@ -2204,7 +2204,8 @@ static void __detach_device(struct iommu_dev_data *dev_data)
 	struct protection_domain *domain;
 	unsigned long flags;
 
-	BUG_ON(!dev_data->domain);
+	if (WARN_ON(!dev_data->domain))
+		return;
 
 	domain = dev_data->domain;