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>