genirq: Annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough. The
fallthrough in __irq_set_trigger() lacks an annotation. Add it.
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190114203154.17125-1-malat@debian.org

kernel/irq/manage.c

@@ -723,6 +723,7 @@ int __irq_set_trigger(struct irq_desc *desc, unsigned long flags)
 	case IRQ_SET_MASK_OK_DONE:
 		irqd_clear(&desc->irq_data, IRQD_TRIGGER_MASK);
 		irqd_set(&desc->irq_data, flags);
+		/* fall through */
 
 	case IRQ_SET_MASK_OK_NOCOPY:
 		flags = irqd_get_trigger_type(&desc->irq_data);