PCI: Clean up ATS error handling

There's no need to BUG() if we enable ATS when it's already enabled.  We
don't need to BUG() when disabling ATS on a device that doesn't support ATS
or if it's already disabled.  If ATS is enabled, certainly we found an ATS
capability in the past, so it should still be there now.

Clean up these error paths.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>

drivers/pci/ats.c

@@ -44,11 +44,12 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
 	u16 ctrl;
 	struct pci_dev *pdev;
 
-	BUG_ON(dev->ats_cap && dev->ats_enabled);
-
 	if (!dev->ats_cap)
 		return -EINVAL;
 
+	if (WARN_ON(pci_ats_enabled(dev)))
+		return -EBUSY;
+
 	if (ps < PCI_ATS_MIN_STU)
 		return -EINVAL;
 
@@ -83,7 +84,8 @@ void pci_disable_ats(struct pci_dev *dev)
 	struct pci_dev *pdev;
 	u16 ctrl;
 
-	BUG_ON(!dev->ats_cap || !dev->ats_enabled);
+	if (WARN_ON(!pci_ats_enabled(dev)))
+		return;
 
 	if (atomic_read(&dev->ats_ref_cnt))
 		return;		/* VFs still enabled */
@@ -107,8 +109,6 @@ void pci_restore_ats_state(struct pci_dev *dev)
 
 	if (!pci_ats_enabled(dev))
 		return;
-	if (!pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ATS))
-		BUG();
 
 	ctrl = PCI_ATS_CTRL_ENABLE;
 	if (!dev->is_virtfn)