tty: cancel delayed work before ldisc and disc_data become NULL

tty->ldisc and tty->disc_data become NULL before worker get
schedule to process the flushing echo buffer with small delays.
This leads to a mem abort is being observed in work item
continue_process_echoes(). Fix cancelling the work item before
tty->ldisc and tty->disc_data become NULL.

tty_release()
  tty_release_struct()
    tty_ldisc_release()
        tty_ldisc_kill()  // here tty->ldisc and tty->disc_data became NULL
    release_tty()
      tty_kref_put()
        kref_put(&tty->kref, queue_release_one_tty)

Change-Id: I168d6c93d011a8d419a397321a948de44f0ec749
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>