drivers: wifi: esp_at: fix premature socket release

This fixes premature socket release. Without this fix, system will crash
because esp_rx thread can still hold reference to the socket in the
following cases:
1. esp_rx thread has got the socket reference but hasn't unref'ed
   via esp_socket_unref.
2. esp_rx thread can still get the socket reference for refcount not
   being zero in esp_socket_ref.

Signed-off-by: Chun-Chieh Li <ccli8@nuvoton.com>

Author: ccli8

drivers/wifi/esp_at/esp_socket.c

@@ -76,6 +76,11 @@ void esp_socket_unref(struct esp_socket *sock)
 		}
 	} while (!atomic_cas(&sock->refcount, ref, ref - 1));
 
+	/* notifies free only on 1-to-0 transition */
+	if (ref > 1) {
+		return;
+	}
+
 	k_sem_give(&sock->sem_free);
 }