wifi: esp_at: notify receivers on async close

`esp_close_work` can be queued from the `on_cmd_closed`, which clears
`ESP_SOCK_CONNECTED` and sets `ESP_SOCK_CLOSE_PENDING`, but does no
further work. The receive callback should still be run with no data when
the socket is closed through this mechanism.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>