spapr/vio: quiet down the "irq" property accessors

commit efe2add7 ("spapr/vio: deprecate the "irq" property")
introduced get/set accessors for the "irq" property to warn of its
usage, but the warning in the get pollutes the monitor 'info qtree'.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>