Record the speed of a USB device based on the port's speed at the time
that the device was enabled.  This allows us to remember the device's
speed even after the device has been disconnected (and so the port's
current speed has changed).

In particular, this allows us to correctly identify the transaction
translator for a low-speed or full-speed device after the device has
been disconnected.

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