smdk4412: Use the USB audio HAL for external devices

 * I recently fixed several issues in the usbaudio HAL, making it able
   to handle arbitrary audio devices connected via OTG.
 * Point the dock outputs to the correct place in order to get audio.

 ** Due to a broken transaction translator in the USB hardware (yay
   Exynos4), you may need to connect certain devices thru a powered USB
   2.0 hub in order for them to work.

Change-Id: I326d257090a1b59293ccbacac26c106e72ce6ee7