cpu-boost: Fix cores getting stuck. Finally.
* In exclusive hotplug system, if a core is not taken down
  explicitly, it is observed that it gets stuck at input
  boost frequency.
* This issue maybe particular to only this kernel but this
  fix resolves it completely.
* Root cause seems to be removal work is lost if queued on
  sibling cores. So simply fix this by queuing on boot CPU.

* Finally, I can use min_cpus_online > 1 without any issue.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>