xen/usb: switch xen-hcd to use INVALID_GRANT_REF

Instead of using a private macro for an invalid grant reference use
the common one.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juergen Gross <jgross@suse.com>