TTY: con3215, use tty from tty_port

Obtain tty_struct only once in ISR and pass it down to
raw3215_next_io. Other than that, we just use the tty with raised
reference. And set it properly in open and close.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux390@de.ibm.com
Cc: linux-s390@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>