s390/smp,sclp: fix size of sclp_cpu_info structure

struct sclp_cpu_info contains entries only for 255 cpus, while the new
smp fallback sigp detection code will fill up to 256 entries.
Even though there is no machine available which has 256 cpus and where
in addition the fallback sigp cpu detection code will be used we better
fix this, to prevent out of bound accesses.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>