drivers: wifi: esp: suppress warning logs when socket was closed

Socket can be closed either by Zephyr or by peer. In the former case ESP
WiFi chip still notfies about closed socket, which currently results in
printing warning log:

  <wrn> wifi_esp: Link X already closed

Change level of this log from warning to debug, so that driver users are
not concerned about situation that is a normal behaviour.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>