ath10k: track vif list internally

mac80211 interface interations functions have
peculiar locking issues. This patch introduces
internal (to ath10k) vif list that will be used
for vif iteration purposes.

kvalo: remove extra INIT_LIST_HEAD()

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>