cable_is_40wire — 40/80/SATA decider
|struct ata_port * ap|
This function encapsulates the policy for speed management in one place. At the moment we don't cache the result but there is a good case for setting ap->cbl to the result when we are called with unknown cables (and figuring out if it impacts hotplug at all).
Return 1 if the cable appears to be 40 wire.