ril: start swrouting only if needed

 * If hwrouting is enabled in the mixer settings ("Voice route" control set to "Voice to twl4030"),
   we won't start the swrouting thread.
 * Swrouting is only started if the "Voice route" control is set to "Voice to SoC".

Signed-off-by: Lukas Märdian <lukas@goldelico.com>