Squashed commit of the following:

    lpc43/54 SDMMC:  Wading through bits trying to make sense out of what is a event want interrupt which deal mostly with commands+responses and what is a data transfer event.  DTO is the only wildcard.  I think this clear now but DMA transfers will fail with CRC error.
    lpc43/54 SDMMC:  DTO is a wait event, not a transfer event.
    lpc43/54 SDMMC:  Significant simplication to previous design.  More debug output.
    lpc43/54 SDMMC:  Don't allow duplicate events in waitmask and xfrmask.
    lpc43/54 SDMMC:  Not all interrupts were being disabled at the end of a transfer.
    lpc43/54 SDMMC:  Defer enabling DMA transfer interrupts until after command has been sent.