PCI: Use 'pci_channel_state_t' instead of 'enum pci_channel_state'

The method struct pci_error_handlers.error_detected() is defined and
documented as taking an 'enum pci_channel_state' for the second argument,
but most drivers use 'pci_channel_state_t' instead.

This 'pci_channel_state_t' is not a typedef for the enum but a typedef for
a bitwise type in order to have better/stricter typechecking.

Consolidate everything by using 'pci_channel_state_t' in the method's
definition, in the related helpers and in the drivers.

Enforce use of 'pci_channel_state_t' by replacing 'enum pci_channel_state'
with an anonymous 'enum'.

Note: Currently, from a typechecking point of view this patch changes
nothing because only the constants defined by the enum are bitwise, not the
enum itself (sparse doesn't have the notion of 'bitwise enum'). This may
change in some not too far future, hence the patch.

[bhelgaas: squash in
  https://lore.kernel.org/r/20200702162651.49526-3-luc.vanoostenryck@gmail.com
  https://lore.kernel.org/r/20200702162651.49526-4-luc.vanoostenryck@gmail.com]
Link: https://lore.kernel.org/r/20200702162651.49526-2-luc.vanoostenryck@gmail.com
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/net/ethernet/intel/ixgb/ixgb_main.c

@@ -82,7 +82,7 @@ static int ixgb_vlan_rx_kill_vid(struct net_device *netdev,
 static void ixgb_restore_vlan(struct ixgb_adapter *adapter);
 
 static pci_ers_result_t ixgb_io_error_detected (struct pci_dev *pdev,
-                             enum pci_channel_state state);
+                             pci_channel_state_t state);
 static pci_ers_result_t ixgb_io_slot_reset (struct pci_dev *pdev);
 static void ixgb_io_resume (struct pci_dev *pdev);
 
@@ -2194,7 +2194,7 @@ ixgb_restore_vlan(struct ixgb_adapter *adapter)
  * a PCI bus error is detected.
  */
 static pci_ers_result_t ixgb_io_error_detected(struct pci_dev *pdev,
-                                               enum pci_channel_state state)
+                                               pci_channel_state_t state)
 {
 	struct net_device *netdev = pci_get_drvdata(pdev);
 	struct ixgb_adapter *adapter = netdev_priv(netdev);