genirq, irq.h: Fix kernel-doc warnings

Fix kernel-doc warnings in linux/irq.h:

  Warning(include/linux/irq.h:201): No description found for parameter 'node'
  Warning(include/linux/irq.h:201): Excess struct/union/enum/typedef member 'cpu' description in 'irq_desc'
  Warning(include/linux/irq.h:434): No description found for parameter 'node'
  Warning(include/linux/irq.h:434): Excess function parameter 'cpu' description in 'alloc_desc_masks'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
LKML-Reference: <4A3467EC.50006@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>