alsa_sound: Improve USB device detection

 * Samsung's proprietary dock was hijacking the routing when a USB audio
   device such as a DAC was connected in host mode.
 * Instead of blindly shoving stuff down a route that doesn't work for
   the dock case, check the switch in sysfs for the proprietary dock
   first.
 * Remove the DOCK_USBAUDIO define entirely and just wrap it up with
   everything else that SAMSUNG_AUDIO includes.

Change-Id: Id9ea2641b67c68a8b38d24aae88b186ee6e991f0