ath6kl: Move and rename ath6kl_cleanup_vif function

Rename ath6kl_cleanup_vif function as 'ath6kl_cfg80211_vif_stop'
which is the more appropriate name considering the functionality
of the module and vif specific cleanup is actually done by
ath6kl_cfg80211_vif_cleanup. Also move it to cfg80211.c.
Also make ath6kl_cfg80211_sta_bmiss_enhance as static function.
This addresses a FIXME/TODO.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>