The EDMA engine cannot tolerate a mix of NCQ/non-NCQ commands,
and cannot be used for PIO at all.  So we need to prevent libata
from trying to feed us such mixtures.

Introduce mv_qc_defer() for this purpose, and use it for all chip versions.
Signed-off-by: NMark Lord <mlord@pobox.com>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>