[usb] Do not call usb_hotplug() when registering a new hub

The action of registering a new hub can itself happen in only two
ways: either a new USB hub has been created (in which case we are
already inside a call to usb_hotplug()), or a new root hub has been
created.

In the former case, we do not need to issue a further call to
usb_hotplug(), since the hub's ports will all be marked as changed and
so will be handled after the return from register_usb_hub() anyway.
Calling usb_hotplug() within register_usb_hub() leads to a confusing
order of events, such as:

- root hub port 1 detects a change
- root hub port 2 detects a change
- usb_hotplug() is called
  - root hub port 1 finds a USB hub
    - usb_hotplug() is called
      - this inner call to usb_hotplug() handles root hub port 2

Fix by calling usb_hotplug() only from usb_step() and from
register_usb_bus().  This avoids recursive calls to usb_hotplug() and
ensures that devices are enumerated in the order of detection.

Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/bus/usb.c

 	 */
 	usb_poll ( bus );
 
-	/* Attach any devices already present */
-	usb_hotplug();
-
 	return 0;
 
 	hub->driver->close ( hub );
 	if ( ( rc = register_usb_hub ( bus->hub ) ) != 0 )
 		goto err_register_hub;
 
+	/* Attach any devices already present */
+	usb_hotplug();
+
 	return 0;
 
 	unregister_usb_hub ( bus->hub );