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Try using $query->get() instead of get_query_var().

function my_orderby_filter($orderby, &$query){
    global $wpdb;
    //figure out whether you want to change the order
    if ($query->get("post_type") == "portfolio") {
         return "$wpdb->posts.post_title ASC";
    }
    return $orderby;
    }

Alternatively, you can use this one, which filters the query via the pre_get_posts hook.

add_action( 'pre_get_posts', 'my_orderby_filter2' );
function my_orderby_filter2( $query ) {
    if ( 'portfolio' === $query->get( 'post_type' ) ) {
        $query->set( 'orderby', 'title' );
        $query->set( 'order', 'ASC' );
    }
}