rtl8712: Replace semaphore terminate_cmdthread_sema with completion

The semaphore 'terminate_cmdthread_sema' is used as completion,
so convert it to a struct completion type.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>