qcacld-3.0: Fix potential OOB memory access

Currently in the wma_stats_ext_event_handler(), the buf_ptr
is not pointing correctly to the event data received from FW.
This is leading to an OOB memory access during qdf_mem_copy().

So, to avoid this issue correctly point the buf_ptr to the event
data sent by the FW in the TLV.

Change-Id: Iffa3e96a6a36eff5899a7a9a7febe0ebb9d7878f
CRs-Fixed: 4011656