s390/sclp: always stay within bounds of the early sccb

Make sure the _sclp_print_lm function stays within bounds of the early
sccb, even if the passed string is very long.  If the string is too
long, the remaining characters will be dropped.

Suggested-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>