Add busdma sync ops before reading and after modifying the descriptor rings.

This was previously working by accident because BUSDMA_COHERENT_MEMORY has
always been set to strongly-ordered on arm.  Now we're moving towards
normal-uncacheable (what might be called write-combining on other platforms)
and using the proper sync ops will be more important.  Of course, that
opens the question of just what is the "proper" sync op for shared
concurrent dma access as opposed to accesses where the handoff of control
of the memory has well-defined sequence points that match the available
busdma sync operations.