* Only call ata_sg_setup{_one} if ATA_QCFLAG_SG is set.  Preparation
  for future use, as currently ATA_QCFLAG_SG is always set when
  ata_qc_issue is called.

  This change in theory is incorrect for Promise TX/SX4 drivers,
  since those drivers set up the Promise-specific packet in their
  ->fill_sg hook, which is now called conditionally.

  A FIXME that doesn't affect anything, for now.

* ATA_PROT_ATAPI and ATA_PROT_ATAPI_DMA command issue need to be
  differentiated.

* Create and use ata_qc_set_polling() to consistently set/clear the
  flags associated with using polling instead of interrupts.