spapr_pci: Drop some dead error handling

chassis_from_bus() uses object_property_get_uint() to get property
"chassis_nr" of the bridge device.  Failure would be a programming
error.  Pass &error_abort, and simplify its callers.

Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200505152926.18877-18-armbru@redhat.com>