s390/3270: readd tty3270_open

Reintroduce the tty3270_open function which has been removed by
git commit 20cda6f2 "TTY: tty3270, add tty install". Without
the open function in the tty_operations tty_open will return
-ENODEV and the 3270 tty will not work.

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