s390x: sclp: Only fetch read info byte 134 if cpu entries are above it

The cpu offset tells us where the cpu entries are in the sclp read
info structure.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>