ath10k: fix WEP in AP and IBSS mode

WEP encoding was not working properly for AP and IBSS mode.
TX frames were encrypted with default WEP tx key index set
always to zero, what sometimes was wrong when different
key index should be used. This patch allows to update
WEP key index also for AP and IBSS mode.
Problem detected during automated WEP tests.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>