serial: core: fix crash in uart_suspend_port

[ Upstream commit 88e2582e ]

With serdev we might end up with serial ports that have no cdev exported
to userspace, as they are used as the bus interface to other devices. In
that case serial_match_port() won't be able to find a matching tty_dev.

Skip the irq wakeup enabling in that case, as serdev will make sure to
keep the port active, as long as there are devices depending on it.

Fixes: 8ee3fde0 (tty_port: register tty ports with serdev bus)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

drivers/tty/serial/serial_core.c

@@ -2026,7 +2026,7 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *uport)
 	mutex_lock(&port->mutex);
 
 	tty_dev = device_find_child(uport->dev, &match, serial_match_port);
-	if (device_may_wakeup(tty_dev)) {
+	if (tty_dev && device_may_wakeup(tty_dev)) {
 		if (!enable_irq_wake(uport->irq))
 			uport->irq_wake = 1;
 		put_device(tty_dev);