ath6kl: Introduce spinlock to protect vif specific information

Use this spinlock to protect the vif's data instead of
one from ath6kl.

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