ath6kl: Fix bug in setting default key index for tx in AP mode

vif->def_txkey_index is set to key_index in ath6kl_cfg80211_add_key().
If the interface is configured with multiple static wep keys,
vif->def_txkey_index would be holding the index of the last key configured,
not the default tx key index. Remove this unnecessary default key index setting
in ath6kl_cfg80211_add_key() to configure the right key index in WEP thereby
make it work when multiple wep keys are configured.

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