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

[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>