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: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/samsung.c

@@ -1803,6 +1803,7 @@ static int __init s3c24xx_serial_modinit(void)
 
 static void __exit s3c24xx_serial_modexit(void)
 {
+	platform_driver_unregister(&samsung_serial_driver);
 	uart_unregister_driver(&s3c24xx_uart_drv);
 }