you're having trouble with the active record functions...
When you use the function
$this->db->get('usuario');
This translates to the query:
SELECT * FROM usuario
So try changing your code to something like this:
$this->db->select('privilegio');
$this->db->from('usuario');
$this->db->where('cod_func',$this->session->userdata('cod_func'));
$this->db->limit(1); //You're only expecting one result
$result = $this->db->get(); // Save the result to the variable result
//Edited for the comment, something like
$result = $result->first_row('array'); //Here I'm fetching only the first row into an array
$privilegio = $result['privilegio']; //Im saving the result from the query in the variable $privilegio
This translates to:
SELECT priviliegio FROM usuario WHERE cod_func = 'some_value' LIMIT 1;
Then you can do whatever you want with the $result variable, please refer to documentation to see what you can do...