thermal: Don't limit CPU frequency after msm_thermal is disabled

There may be an outstanding task to check for threshold crossing
when msm_thermal is disabled and CPU is restored to maximum
frequency. This task can get scheduled after the maximum frequency
is restored, and result in another mitigation that pins CPU
frequencies to 918mHz. This happens in the case that the final
temperature was still above the max temperature threshold.
This patch ensures any outstanding tasks are cancelled and flushed
before finally restoring the CPU frequencies.

CRs-fixed: 351614
Change-Id: Ifce00b379989b78856803ac09c7403207f5c998b
Signed-off-by: Eugene Seah <eseah@codeaurora.org>