spapr: Use appropriate getter for PC_DIMM_SLOT_PROP

The PC_DIMM_SLOT_PROP property is defined as:

    DEFINE_PROP_INT32(PC_DIMM_SLOT_PROP, PCDIMMDevice, slot,
                      PC_DIMM_UNASSIGNED_SLOT),

Use object_property_get_int() instead of object_property_get_uint().
Since spapr_memory_plug() only gets called if pc_dimm_pre_plug()
succeeded, we expect to have a valid >= 0 slot number, either because
the user passed a valid slot number or because pc_dimm_get_free_slot()
picked one up for us.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160309730758.2739814.15821922745424652642.stgit@bahia.lan>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>