TTY: ehv_bytechan: 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>

drivers/tty/ehv_bytechan.c

@@ -859,6 +859,7 @@ error:
  */
 static void __exit ehv_bc_exit(void)
 {
+	platform_driver_unregister(&ehv_bc_tty_driver);
 	tty_unregister_driver(ehv_bc_driver);
 	put_tty_driver(ehv_bc_driver);
 	kfree(bcs);