net: wifi: Add disconnection success code

When disconnect request is successful, the status
was taking as WIFI_REASON_DISCONN_UNSPECIFIED only.
Adding WIFI_REASON_DISCONN_SUCCESS which can be help
to determine status of disconnect request. If this status
is failed then reason can be useful.

Signed-off-by: Kapil Bhatt <kapil.bhatt@nordicsemi.no>