Change Allowed Value in list in Drupal 7

<?php
/**
* Implements hook_permission().
*/
function dynallowed_permission() {
  return array(
    'administer dynallowed' => array(
      'title' => t('Alter Field List Allowed Values Function'),
      'description' => t('This give field administrators access to alter the options list source for fields.'),
    ),
  );
}
function dynallowed_form_field_ui_field_settings_form_alter(&$form, &$form_state){
  dynallowed_form_field_ui_field_edit_form_alter($form, $form_state);
}
function dynallowed_form_field_ui_field_edit_form_alter(&$form, &$form_state) {
  if (user_access('administer dynallowed')) {
    $form['field']['settings']['allowed_values_function'] = array(
      '#type' => 'select',
      '#title' => t('Allowed value function'),
      '#default_value' => $form['field']['settings']['allowed_values_function']['#value'],
      '#options' => dynallowed_allowed_functions_callbacks(),
      '#empty_value' => '',
      '#empty_option' => t('Allowed values list'),
    );
  }
}

function dynallowed_allowed_functions_callbacks() {
  $options = array(
    'allowed_values_function' => t('Dynamic Value From function'),
  );
  return $options;
}

function allowed_values_function(){
// Write Your Code Here
return array(
    'AF'=>'Afghanistan',
    'AL'=>'Albania',
    'DZ'=>'Algeria',
    'AS'=>'American',
    'AD'=>'Andorra',
    'AO'=>'Angola',
  );
}