USB: HSO: make tty_operations const

As recommended by Arjan.

Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Andrew Bird <ajb@spheresystems.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>