usb: Set unused ports to "fixed" rather than "unknown"

The Microsoft document "Using ACPI to Configure USB Ports on a Computer"
makes it clear that the removable flag will be cleared on ports that are
marked as unused by the firmware. Handle this case to match.

Signed-off-by: Matthew Garrett <mjg59@coreos.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>