ath6kl: Use interface index from wmi data headr

Interface index is passed in wmi data header as well, use it
to get the corresponding vif structure.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>