wcn36xx: Don't use the destroyed hal_mutex

ieee80211_unregister_hw() might invoke operations to stop the interface,
that uses the hal_mutex. So don't destroy it until after we're done
using it.

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