sata_nv: Cleanup taskfile setup

This edits the taskfile setup to more closely match the way that libata
sends the taskfile for other controllers. This avoids putting taskfile writes
into the CPB buffer that are not needed according to the taskfile flags.

Signed-off-by: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>