ns9xxx: fix handle_prio_irq to unmask irqs with lower priority

When an irq is reported all lower prio irqs are masked until the current
irq is acked.  So never leave handle_prio_irq without acking.

desc->status & IRQ_INPROGRESS should never become true because the current
irq is masked until it is acked, too.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>