x86/apic/uv: Fix the hotplug notifier

The notifier is missing the CPU_DOWN_FAILED transition. That leaves the
heartbeat disabled when CPU_DOWN_PREPARE fails.

It also does not handle the FROZEN transition variants. That might not be an
issue for UV, but it's inconsistent.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Dimitri Sivanich <sivanich@sgi.com>