hso: fix refcounting on the ttyHSx devices

The references on ttyHSx devices were not decremented correctly when
the tty was closed. The helper freeing the serial devices was never
called because of that, and the module left some dangling sysfs
devices after being unloaded.

Signed-off-by: Olivier Blin <blino@mandriva.com>
Cc: Jari Tenhunen <jari.tenhunen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>