Fix sample offset of adc channels.

   This fixes a problem where the adc samples over the channel interface
   are misaligned. So before, instead of:

     (l1, r1) ... (l2, r2) ... (l3, r3) ...

   you would get

     (r1, l2) ... (r2, l3) ... (r3, l4) ...

   This fixes the problem by rotating the main loop so the ports I/O syncs
   with the channel I/O.