sata_mv: workaround errata PCI#7

Workaround for 60x1-B2 errata PCI#7.

Write-combining may be unreliable when chip operates in PCI-X mode,
so disable write-combining when in PCI-X mode.

Also, update the errata comments at the top of sata_mv,
and include a note about errata PCI#11.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>