s390/sclp: keep sclp_early_sccb

Keep sclp_early_sccb so it can also be used after initdata has been
freed. This is a prerequisite to allow printing a message from the
machine check handler.

Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>