serdev: ttyport: check whether tty_init_dev() fails

My static checker complains that we don't have any error handling here.
It's simple enough to add it.

Fixes: bed35c6d ("serdev: add a tty port controller driver")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serdev/serdev-ttyport.c

@@ -97,6 +97,8 @@ static int ttyport_open(struct serdev_controller *ctrl)
 	struct ktermios ktermios;
 
 	tty = tty_init_dev(serport->tty_drv, serport->tty_idx);
+	if (IS_ERR(tty))
+		return PTR_ERR(tty);
 	serport->tty = tty;
 
 	serport->port->client_ops = &client_ops;