s390/sclp: simplify early hsa_size detection

Architecture documentation suggests that hsa_size has been available in
the read info since the list-directed ipl dump has been introduced. By
using this value few early sclp calls could be avoided.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>