s due to the fact that when the tty is hung up,
tty_port_close_start() returns always zero, and acm_tty_close() never reaches
the point where acm_tty_unregister() is called.

Here is a fix for this.

Signed-off-by: Francesco Lavra <francescolavra@interfree.it>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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