genirq: Fix inverted condition in handle_nested_irq()

Marek reported that the rework of handle_nested_irq() introduced a inverted
condition, which prevents handling of interrupts. Fix it up.

Fixes: 2ef2e130 ("genirq/chip: Rework handle_nested_irq()")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Closes: https://lore.kernel/org/all/46ed4040-ca11-4157-8bd7-13c04c113734@samsung.com