libata: power usage patches

Hi,
The libata core leaves the phy powered on for unoccupied ports, costing us about .75 Watts of extra power usage. If the port is not hotpluggable, there's no reason to leave the port powered on if there are no devices attached after initial probe.

As I was investigating this issue I also ran across a bug in the ahci driver - it sets PxCMD.ST without checking to see if any devices are attached. thanks,
Kristen