ath6kl: Protect ath6kl_cfg80211_vif_cleanup using rtnl_locks

ath6kl_cfg80211_vif_cleanup calls 'unregister_netdevice' which
inturn calls 'unregister_netdevice_queue' and it requires holding
rtnl_lock semaphore protection.

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