s390/3270: fix view reference counting

The tty3270_close function set tty->driver_data to NULL after the
open count dropped to zero but tty3270_cleanup still needs the
pointer to the tty3270 view to drop the reference count.

Move the clearing of tty->driver_data to tty3270_cleanup and
drop the unnecessary tty_port_tty_set calls in tty3270_install.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>