[usb] Detect missed disconnections

The USB core will currently fail to detect disconnections if a new
device has attached by the time the port is examined in
usb_hotplug().

Fix by recording the fact that a disconnection has taken place
whenever the "connection status changed" (CSC) bit is observed to be
set.  (Whether the change represents a disconnection or a
reconnection, it indicates that the port has experienced some time of
being disconnected.)

Note that the time at which a disconnection can be detected varies by
hub type.  In particular: root hubs can observe the CSC bit when
polling, and so will record the disconnection before calling
usb_port_changed(), but USB hubs read the port status (and hence the
CSC bit) only during the call to hub_speed(), long after the call to
usb_port_changed().

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

src/include/ipxe/usb.h

@@ -747,6 +747,12 @@ struct usb_port {
 	unsigned int protocol;
 	/** Port speed */
 	unsigned int speed;
+	/** Port disconnection has been detected
+	 *
+	 * This should be set whenever the underlying hardware reports
+	 * a connection status change.
+	 */
+	int disconnected;
 	/** Port has an attached device */
 	int attached;
 	/** Currently attached device (if in use)