wcn36xx: Clear encrypt_type when deleting bss key

This fixes a problem connecting to an open network after being
connected to an encrypted network.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>