s390/mem_detect: rely on diag260() if sclp_early_get_memsize() fails

In case sclp_early_get_memsize() fails but diag260() succeeds make sure
some sane value is returned. This error scenario is highly unlikely,
but this change makes system able to boot in such case.

Suggested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>