x86/irq: Check for VECTOR_UNUSED directly

It's simpler and more intuitive to directly check for VECTOR_UNUSED than
checking whether the other error codes are not set.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/caeaca93-5ee1-cea1-8894-3aa0d5b19241@gmail.com