[usb] Allow additional settling time for out-of-spec hubs
Some hubs (e.g. the Avocent Corp. Virtual Hub on a Lenovo x3550
Integrated Management Module) have been observed to require more than
the standard 200ms for ports to stabilise, with the result that
devices appear to disconnect and immediately reconnect during the
initial bus enumeration.
Work around this problem by allowing specific hubs an extra 500ms of
settling time.
Signed-off-by: Michael Brown <mcb30@ipxe.org>