genirq: Add missing comment for IRQD_STARTED

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20170619235444.614913014@linutronix.de

include/linux/irq.h

@@ -200,6 +200,7 @@ struct irq_data {
  * IRQD_WAKEUP_ARMED		- Wakeup mode armed
  * IRQD_FORWARDED_TO_VCPU	- The interrupt is forwarded to a VCPU
  * IRQD_AFFINITY_MANAGED	- Affinity is auto-managed by the kernel
+ * IRQD_IRQ_STARTED		- Startup state of the interrupt
  */
 enum {
 	IRQD_TRIGGER_MASK		= 0xf,