[usb] Maintain single lists of halted endpoints and changed ports

When an EHCI hotplug action results in the controller disowning the
port, it will result in a hotplug action on the corresponding UHCI or
OHCI controller.  Allow such hotplug actions to be carried out as part
of the same call to usb_step() or usb_register_bus(), by maintaining a
single central list of changed ports.

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

src/include/ipxe/usb.h

@@ -922,12 +922,6 @@ struct usb_bus {
 	struct list_head devices;
 	/** List of hubs */
 	struct list_head hubs;
-	/** List of changed ports */
-	struct list_head changed;
-	/** List of halted endpoints */
-	struct list_head halted;
-	/** Process */
-	struct process process;
 
 	/** Host controller operations */
 	struct usb_bus_host_operations *host;