Updating all rows of table with $wpdb

It will not work as the update statement requires a selector to narrow down what to update. You should use a general query. For your needs use this:

$wpdb->query( 
    $wpdb->prepare( 
        "UPDATE $wpdb->comments
         SET `comment_karma` = %s",
         '123'
    )
);

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)