msm: cpufreq: Fix the race between dbs_refresh and hotplug.

The demand based switching responds to input events and schedules
work items on all online cpus to increase the frequency. This will
call the dbs_refresh_callback on respective cpu. If this work item
kicks in while cpu1 is progressing its way through cpufreq_add_dev,
there is a chance that work item grabs the cpu_policy_rwsem of cpu1
and relases that of cpu0, when add device is working through releas-
ing lock of cpu1 and reaquiring that of cpu0. This is locking out
cpu1 when it tries to come up online next time. The fix is hold the
hot plug lock while processing refresh work items.

CRs-Fixed: 362444
Change-Id: I57db7c7a9b58369bf059302896f3bf9eca2bc2bb
Signed-off-by: Krishna Vanka <kvanka@codeaurora.org>