qcacmn: Avoid buffer overflow in roam scan stats extract handler

In extract_roam_scan_stats_res_evt_tlv(), there is potential
buffer-overflow due to no input validation of following event
parameters from firmware:
(a) Roam scan frequencies against maximum value of 50
(WMI_ROAM_SCAN_STATS_CHANNELS_MAX) and
(b) Roam scan candidates against maximum value of 4
(WMI_ROAM_SCAN_STATS_CANDIDATES_MAX)

To fix this, validate roam scan stats event parameters.

Change-Id: I866b492f7ccb48c4960ff25a9e817cbdb394509e
CRs-Fixed: 2348299