ipack: ipoctal: fix tty registration race

Make sure to set the tty class-device driver data before registering the
tty to avoid having a racing open() dereference a NULL pointer.

Fixes: 9c1d784 ("Staging: ipack/devices/ipoctal: Get rid of ipoctal_list.")
Cc: stable@vger.kernel.org      # 3.7
Acked-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210917114622.5412-3-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: jhovold

drivers/ipack/devices/ipoctal.c

@@ -392,13 +392,13 @@ static int ipoctal_inst_slot(struct ipoctal *ipoctal, unsigned int bus_nr,
 		spin_lock_init(&channel->lock);
 		channel->pointer_read = 0;
 		channel->pointer_write = 0;
-		tty_dev = tty_port_register_device(&channel->tty_port, tty, i, NULL);
+		tty_dev = tty_port_register_device_attr(&channel->tty_port, tty,
+							i, NULL, channel, NULL);
 		if (IS_ERR(tty_dev)) {
 			dev_err(&ipoctal->dev->dev, "Failed to register tty device.\n");
 			tty_port_destroy(&channel->tty_port);
 			continue;
 		}
-		dev_set_drvdata(tty_dev, channel);
 	}
 
 	/*