x86/irq: Improve definition of VECTOR_SHUTDOWN et al

These values are used with IS_ERR(), so it's more intuitive to define
them like a standard PTR_ERR() of a negative errno.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/146835e8-c086-4e85-7ece-bcba6795e6db@gmail.com