In part of the effort to remove SCSI specifics from the libata
internals, remove references to cmd->use_sg.  cmd->use_sg
becomes ATA_QCFLAG_SG, and !cmd->use_sg becomes ATA_QCFLAG_SINGLE.

Convenience constant ATA_QCFLAG_DMAMAP is created when the programmer
wishes to refer collectively to ATA_QCFLAG_{SG,SINGLE}.