cpufreq: Use correct locking for cpufreq_cpu_data

Use write lock when updating cpufreq_cpu_data,
and read lock when getting the policy pointer.

CRs-Fixed: 689522
Change-Id: I454f0d575157b3411d369e04097386f50aeaaa1c
Signed-off-by: Maria Yu <aiquny@codeaurora.org>