net: socket_service: Add underscore in the idx variable name

Nothing critical, but it just looks better if the service name is
separated from the prefix, i.e:
    _z_net_socket_service_idx_udp_service_async
vs current:
    _z_net_socket_service_idxudp_service_async

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>