genirq: Fix function documentation of __irq_alloc_descs()

The function got renamed at some point, but the kernel-doc was not updated.

Signed-off-by: Liguang Zhang <zhangliguang@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1573656093-8643-1-git-send-email-zhangliguang@linux.alibaba.com