spapr_pci: s/INT64_MAX/UINT64_MAX/

spapr_pci: s/INT64_MAX/UINT64_MAX/

It doesn't make sense for a region to be INT64_MAX in size:
memory core uses UINT64_MAX as a special value meaning
"all 64 bit" this is what was meant here.

While this should never affect the spapr system which at the moment always
has < 63 bit size, this makes us hit all kind of corner case bugs with
sub-pages, so users are probably better off if we just use UINT64_MAX
instead.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Alexander Graf <agraf@suse.de>