thermal: Fix hotplug race condition leading to memory corruption.

msm_thermal module made a copy of the CPUFreq policy structure
pointer but continues to try and use it after the core has been
hotplugged (and the original structure is freed). The fix is to
use cpufreq_cpu_get/put instead of copying cpufreq pointers.

CRs-fixed: 342351
Change-Id: I312d8bccc09a5df5805f99fecdc9409d9d4ed2be
Signed-off-by: Eugene Seah <eseah@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>