acpi: fix "buggy BIOS check" when CPUs are hot removed

Fixes a BUG in ACPI hotplugging.

processor_device_array[pr->id] needs to be set to NULL when removing a CPU.
Else the "buggy BIOS check" in acpi_processor_start mistakenly fires when a
CPU is removed from the system and then later re-added.

Signed-off-by: Alok N Kataria <akataria@vmware.com>
Signed-off-by: Dan Arai <arai@vmware.com>
Cc: Len Brown <lenb@kernel.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>