[usb] Record USB device speed separately from current port speed

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>

src/include/ipxe/usb.h

@@ -698,6 +698,8 @@ struct usb_device {
 	char name[32];
 	/** USB port */
 	struct usb_port *port;
+	/** Device speed */
+	unsigned int speed;
 	/** List of devices on this bus */
 	struct list_head list;
 	/** Device address, if assigned */