modules: hostap: Use net_mgmt context

Using a separate workqueue causes issues without any special locking to
synchronize with networking threads e.g., interface being removed while
the workqueue is trying to synchronize with WPA supplicant.

It's easier to use the net_mgmt thread which is in better sync with
networking.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>