irq.h fix kernel-doc

Add kernel-doc entry in <linux/irq.h> for:

Warning(linux-2.6.22-git12//include/linux/irq.h:177): No description found for parameter 'last_unhandled'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/irq.h

@@ -140,6 +140,7 @@ struct irq_chip {
  * @wake_depth:		enable depth, for multiple set_irq_wake() callers
  * @irq_count:		stats field to detect stalled irqs
  * @irqs_unhandled:	stats field for spurious unhandled interrupts
+ * @last_unhandled:	aging timer for unhandled count
  * @lock:		locking for SMP
  * @affinity:		IRQ affinity on SMP
  * @cpu:		cpu index useful for balancing