wifi: esp_at: sockets not cleaned up on close

[JordanYates]

Describe the bug

The ESP_SOCK_IN_USE flag is never cleared when a socket is closed.
Therefore after ESP_MAX_SOCKETS (5) sockets are opened and closed, esp_socket_get will fail to allocate another socket.

Expected behavior
Closing a socket should result in a complete cleanup of all resources, allowing it to be opened again at a later time.

Environment (please complete the following information):

• Zephyr v2.7.1
• Zephyr v3.0.0

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.