s390/sclp_early: Return correct HSA block count also for zero

Currently we return a negative block count if SCLP returns HSA block
count zero. The reason is that we subtract one for the header page.

So fix this and correctly return zero block count if SCLP returns zero.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>