winc1500: WIFI_DISCONNECT request doesn't raise DISCONNECT_RESULT event

Internal flag (w1500_data.connecting) was not being set to false after
connection. Interface raises NET_EVENT_WIFI_CONNECT_RESULT event with
error status instead of NET_EVENT_WIFI_DISCONNECT_RESULT when
disconnection is manually requested (NET_REQUEST_WIFI_DISCONNECT).

Signed-off-by: Diogo Correia <diogo.correia@fraunhofer.pt>