ath10k: zero arvif memory on add_interface()

The private memory area in vif provided by
mac80211 isn't guaranteed to be zeroed.

This patch should fix issues when switching
between STA and AP interface types.

The tim_bitmap could become polluted by STA bssid
field (since it's a union), wep_keys array
could also become polluted with invalid pointers
and probably much more.

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