serial: 8250_exar: Avoid NULL pointer dereference at ->exit()

It's possible that during ->exit() the private_data is NULL,
for instance when there was no GPIO device instantiated.
Due to this we may not dereference it. Add a respective check.

Note, for now ->exit() only makes sense when GPIO device
was instantiated, that's why we may use the check for entire
function.

Fixes: 81171e7d31a6 ("serial: 8250_exar: Constify the software nodes")
Reported-by: Maxim Levitsky <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>
Tested-by: Maxim Levitsky <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_exar.c

···
 {
 	struct exar8250 *priv = pci_get_drvdata(pcidev);
 	struct uart_8250_port *port = serial8250_get_port(priv->line[0]);
-	struct platform_device *pdev = port->port.private_data;
+	struct platform_device *pdev;
+
+	pdev = port->port.private_data;
+	if (!pdev)
+		return;
 
 	device_remove_software_node(&pdev->dev);
 	platform_device_unregister(pdev);