serial: sunsu: add missing platform_driver_unregister() when module exit

We have registered platform driver when module init, and
need unregister it when module exit.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/tty/serial/sunsu.c

@@ -1592,6 +1592,7 @@ static int __init sunsu_init(void)
 
 static void __exit sunsu_exit(void)
 {
+	platform_driver_unregister(&su_driver);
 	if (sunsu_reg.nr)
 		sunserial_unregister_minors(&sunsu_reg, sunsu_reg.nr);
 }