genirq: Update the comment for generic_handle_irq_desc

genirq: Update the comment for generic_handle_irq_desc

__do_IRQ() was removed by commit 1c77ff2 "genirq: Remove __do_IRQ",
but the comment referring to __do_IRQ() was left.

Update the comment for generic_handle_irq_desc().

Signed-off-by: Huang Shijie <shijie.huang@arm.com>
Cc: jiang.liu@linux.intel.com
Cc: peterz@infradead.org
Cc: rafael.j.wysocki@intel.com
Cc: jason@lakedaemon.net
Cc: marc.zyngier@arm.com
Link: http://lkml.kernel.org/r/1441074950-3893-1-git-send-email-shijie.huang@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>