serial: samsung: 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/serial/samsung.c b/drivers/tty/serial/samsung.c
index 074b919..8942941 100644
--- a/drivers/tty/serial/samsung.c
+++ b/drivers/tty/serial/samsung.c
@@ -1803,6 +1803,7 @@
 
 static void __exit s3c24xx_serial_modexit(void)
 {
+	platform_driver_unregister(&samsung_serial_driver);
 	uart_unregister_driver(&s3c24xx_uart_drv);
 }