Htaccess değil fakat php ile şu şekilde çözmüştüm
<?php

$iphone = strpos($_SERVER['HTTP_USER_AGENT'],"iPhone");
$android = strpos($_SERVER['HTTP_USER_AGENT'],"Android");
$palmpre = strpos($_SERVER['HTTP_USER_AGENT'],"webOS");
$berry = strpos($_SERVER['HTTP_USER_AGENT'],"BlackBerry");
$ipod = strpos($_SERVER['HTTP_USER_AGENT'],"iPod");

if ($iphone || $android || $palmpre || $ipod || $berry == true)
{

    echo '<meta http-equiv="refresh" content="0;url=http://'.$protocol.''.$_SERVER['HTTP_HOST'].'/m'.$_SERVER['REQUEST_URI'].'">';
    
}

?>