Use the return value of spapr_irq_findone() and spapr_irq_claim()
to detect failures. This allows to reduce the error propagation
overhead.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20200914123505.612812-8-groug@kaod.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>