genirq: Fix wrong name in request_percpu_nmi() description

ready_percpu_nmi() was the previous name of prepare_percpu_nmi(). Update
request_percpu_nmi() comment with the correct function name.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Reported-by: Li Wei <liwei391@huawei.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>