wcn36xx: Fix WEP encryption

In case of WEP encryption, driver has to configure shared key for
associated station(s). Note that sta pointer is NULL in case of non
pairwise key, causing NULL pointer dereference with existing code
(sta_priv->is_data_encrypted). Fix this by using associated sta list
instead. This enables WEP support as client, WEP AP is non-functional.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>