irq/irqdomain: Fix comment typo

Fix typo in the comment on top of __irq_domain_add().

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1562388072-23492-1-git-send-email-yuzenghui@huawei.com