[netdevice] Make all net_driver methods optional

Most network upper-layer drivers do not implement all three methods
(probe, notify, and remove).  Save code by making all methods
optional.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/vlan.c

@@ -439,16 +439,6 @@ int vlan_destroy ( struct net_device *netdev ) {
 	return 0;
 }
 
-/**
- * Do nothing
- *
- * @v trunk		Trunk network device
- * @ret rc		Return status code
- */
-static int vlan_probe ( struct net_device *trunk __unused ) {
-	return 0;
-}
-
 /**
  * Handle trunk network device link state change
  *
@@ -505,7 +495,6 @@ static void vlan_remove ( struct net_device *trunk ) {
 /** VLAN driver */
 struct net_driver vlan_driver __net_driver = {
 	.name = "VLAN",
-	.probe = vlan_probe,
 	.notify = vlan_notify,
 	.remove = vlan_remove,
 };