genirq: fix comment to say IRQ_WAKE_THREAD

Trying to implement a driver to use threaded irqs, I was confused when the 
return value to use that was described in the comment above 
request_threaded_irq was not defined.

Turns out that the enum is IRQ_WAKE_THREAD where as the comment said 
IRQ_THREAD_WAKE.

[Impact: do not confuse developers with wrong comments ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <alpine.DEB.2.00.0905121431020.13338@gandalf.stny.rr.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/manage.c

@@ -851,7 +851,7 @@ EXPORT_SYMBOL(free_irq);
  *	still called in hard interrupt context and has to check
  *	whether the interrupt originates from the device. If yes it
  *	needs to disable the interrupt on the device and return
- *	IRQ_THREAD_WAKE which will wake up the handler thread and run
+ *	IRQ_WAKE_THREAD which will wake up the handler thread and run
  *	@thread_fn. This split handler design is necessary to support
  *	shared interrupts.
  *