now uses the DMACSR_READ bit and no longer keeps _nd_dmadir in softc
unified transfer cleanup code, now in routine next_dma_finish_xfer()
fixed bounds checking on registers after transfer.
removed checking for bus errors since the bit is always set on some nexts
(specifically, on mourning, a 25mhz 68040 mono slab)
fixed a couple of dma bugs involving chaining dma buffers.