ahci: Turn off DMA engines when there's no device attached

According to section 10.3.1 of the AHCI spec, PxCMD.ST must not be set
unless there's a device attached. Following this saves us a measurable
quantity of power and does not impair hotplug support. Based on a patch
by Kristen Carlson Accardi.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>