[patch] enable AHCI device initiated power management

Hi,

this patch
http://leaf.dragonflybsd.org/~hofmann/ahci_dipm.diff
enables device initiated AHCI link power management.

Some disks (e.g. the Intel X25-M SSD) don't support host initiated
power management.
Therefore also enable device initiated power management when
ahci power management is set to "aggressive".
This saves about 0.5W on a Thinkpad with Intel X25-M.

 * Move the currently unused function ahci_pm_set_feature() to ahci.c
   and make it work for ATA_SATAFT_DEVIPS.

 * Use ahci_set_feature() to enable ATA_SATAFT_DEVIPS when AHCI power
   management is set to "aggressive".

 * Set ap->link_pwr_mgmt early in ahci_port_link_pwr_mgmt() to ensure
   it is set when ahci_set_feature() -> ahci_poll() -> ahci_port_intr()
   is called.

Inspired-by: Linux


Cheers,
Johannes