msm: hotplug: Prevent concurrency of up_work & down_work

* This change is under test.
* We are calling online_cpu() and offline_cpu() from
  many places.
* There is high chance that up_work and down_work
  used by online_cpu() and offline_cpu() resp called
  concurrently, so make sure to wait for another to
  complete before queuing work.

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