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>
diff --git a/drivers/tty/serial/sunsu.c b/drivers/tty/serial/sunsu.c
index 451687c..0d84657 100644
--- a/drivers/tty/serial/sunsu.c
+++ b/drivers/tty/serial/sunsu.c
@@ -1592,6 +1592,7 @@
 
 static void __exit sunsu_exit(void)
 {
+	platform_driver_unregister(&su_driver);
 	if (sunsu_reg.nr)
 		sunserial_unregister_minors(&sunsu_reg, sunsu_reg.nr);
 }