NET: wan/x25, fix ldisc->open retval

We should never return positive values from ldisc->open, tty layer
doesn't (and never did) expect that.

If we do that, weird things like warnings in tty_ldisc_close happen.

Not sure if dev->base_addr is used somehow now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Andrew Hendry <andrew.hendry@gmail.com>
Cc: linux-x25@vger.kernel.org
Tested-by: Sergey Lapin <slapin@ossfans.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>