The various ways you can send data to/from your ATA device is
known as the ATA taskfile protocol:  PIO single sector, PIO
multiple sector, DMA, DMA TCQ, DMA NCQ, ...

Prior to this change, the data direction (read/write) was encoded
implicitly into the ATA_PROT_xxx value itself.  This increased
complexity in some areas, and inhibited flexibility in others.

This change separates data direction from taskfile protocol, and also
moves the data direction flag (ATA_QCFLAG_WRITE) down to a lower
level (ATA_TFLAG_WRITE).