spapr: Handle freeing of multiple irqs in frontend only

spapr: Handle freeing of multiple irqs in frontend only

spapr_irq_free() can be used to free multiple irqs at once. That's useful
for its callers, but there's no need to make the individual backend hooks
handle this.  We can loop across the irqs in spapr_irq_free() itself and
have the hooks just do one at time.

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