x86/mce/amd: Sanitize thresholding device creation hotplug path

Drop the stupid threshold_init_device() initcall iterating over all
online CPUs in favor of properly setting up everything on the CPU
hotplug path, when each CPU's callback is invoked.

 [ bp: Write commit message. ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200403161943.1458-5-bp@alien8.de