Update a single field when saving an entity
Give a try with field_sql_storage_field_storage_write.

See this blog post with an example:

//Get the id of your field
$name = 'name_of_your_field';
$info = field_info_field($field_name);
$fields = array(info['id']); 

//Execute the storage function
field_sql_storage_field_storage_write('model', $entity, 'update', $fields);