printk: drop redundant devkmsg_log_str memsets

We copy in null terminated strings "on" and "off", no
need to zero out devkmsg_log_str in control_devkmsg().

Link: http://lkml.kernel.org/r/20180119043901.1728-1-sergey.senozhatsky@gmail.com
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>