ath6kl: Remove erroneous flag clearing

WLAN_ENABLED is vif specific, not part of
the driver's struct ath6kl. Proper clearing
of this flag is already taken care in
ath6kl_cleanup_vif.

Cc: wei-jen jlin <jenlin@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>