qcacld-3.0: Move call to hdd power stats callback inside lock

Currently when host receives power stats from fw, before
processing these stats it check for the hdd callback and
processes these stats only if hdd callback is registered.
This callback is checked in
wma_unified_power_debug_stats_event_handler function before
processing of the fw event and outside of any lock. After
processing stats, this hdd cb is invoked, there is an issue
with this approach that while processing these stats if timeout
happens in hdd then hdd sets its callback as NULL and now after
processing stats wma handler invokes this hdd cb which is already
NULL, leading to null pointer dereference.

To avoid above issue, add sme callback and move null check for
hdd cb and call to hdd cb inside sme lock which is the same lock
used while setting hdd cb to null on timeout.

Change-Id: I2203083a17ca3404c5c5af98ddc48ba127e6aee2
CRs-Fixed: 2950633