s390/sclp: prepare smp_fill_possible_mask for global "struct sclp"

We need to rename sclp -> sclp_max to prepare for using the global variable
"sclp" for sclp access later in this function.

Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>