msm: thermal: Don't initialize hotplug thread when not configured

Do not initialize the hotplug thread if the device tree
entry for hotplug thresholds are missing for a target.

CRs-fixed: 580249
Change-Id: I8ed99a757a2bd66a6a669ad78d7c4fe632b56012
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>