sched/core: Fix race condition in clearing hmp request

There is a race condition between clearing an HMP request for active
migration and the actual active migration. Active migration can he
half-way through doing the migration when the HMP request can be cleared
by another core. Move clearing of HMP request to the stopper thread to
avoid this.

Change-Id: I6d73b8f246ae3754ab60984af198333fd284ae16
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>