Because invoking function hdd_medium_assess_init() and
hdd_medium_assess_deinit() are not paired with event
the eSAP_START_BSS_EVENT and eSAP_STOP_BSS_EVENT.
To address this, move hdd_medium_assess_init() to function
__wlan_hdd_cfg80211_start_ap().
Change-Id: I0f778469f0d0fa35089ab0ee884892743935c36e
CRs-Fixed: 2927454