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>