sata_mv ncq Add want ncq parameter for EDMA configuration

An extra EDMA config bit is required for NCQ operation.
So set/clear it as needed, and cache current setting in port_priv.
For now though, it will always be "off" (0).

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>