platform-msi: Fix typos in comment

Fix up one typos @nev -> @nr_irqs.

Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1589770859-19340-1-git-send-email-zhangshaokun@hisilicon.com