Fix for SPI_DMA

spi_dma_read/write_completed were looking at how many transfers were remaining in the NEXT pointer.

They should have been checking the status register which has a bit indicating that the current and next counts are zero.

The result of this was that spi_dma_completed returned true while it was still processing the current buffer, resulting in crap waveforms.

If you approve this then I'll update treetap to point to this.

Cheers,
--Mike