TTY: 68328serial, use tty from tty_port

And refcount that properly.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>