Staging: ipack/devices/ipoctal: fix dereference NULL pointer

After opening and closing the file /dev/ipoctal.X.Y.Z for the second time, it
gives a kernel oops due to a dereference of a NULL pointer.

The problem was that tty->driver_data was not properly initialized when
accessing the file for the second time.

Reported-by: Alberto Garcia Gonzalez <agarcia@igalia.com>
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>