[S390] sclp_async: potential buffer overflow

"len" hasn't been properly range checked so we shouldn't use it as an
array offset.  This can only be written to by root but it would still be
annoying to accidentally write more than 3 characters and corrupt your
memory.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>