Fix various bugs with freeing static DMA allocations in sound drivers:
- Don't compare the DMA map to NULL to determine if bus_dmamap_unload()
  should be called when releasing a static allocation.  Instead, compare
  the bus address against 0.
- Don't assume that the DMA map for static allocations is NULL.  Instead,
  save the value set by bus_dmamem_alloc() so it can later be passed to
  bus_dmamem_free().  Also, add missing calls to bus_dmamap_unload() in
  these cases before freeing the buffer.
- Use the bus address from the bus_dma callback instead of calling
  vtophys() on the address allocated by bus_dmamem_alloc().

Reviewed by:	kan