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: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/tty3270.c

@@ -925,6 +925,20 @@ static int tty3270_install(struct tty_driver *driver, struct tty_struct *tty)
 	return 0;
 }
 
+/*
+ * This routine is called whenever a 3270 tty is opened.
+ */
+static int
+tty3270_open(struct tty_struct *tty, struct file *filp)
+{
+	struct tty3270 *tp = tty->driver_data;
+	struct tty_port *port = &tp->port;
+
+	port->count++;
+	tty_port_tty_set(port, tty);
+	return 0;
+}
+
 /*
  * This routine is called when the 3270 tty is closed. We wait
  * for the remaining request to be completed. Then we clean up.
@@ -1753,6 +1767,7 @@ static long tty3270_compat_ioctl(struct tty_struct *tty,
 static const struct tty_operations tty3270_ops = {
 	.install = tty3270_install,
 	.cleanup = tty3270_cleanup,
+	.open = tty3270_open,
 	.close = tty3270_close,
 	.write = tty3270_write,
 	.put_char = tty3270_put_char,