libata: tell scsi layer device supports runtime power off

If ATA device supports "Device Attention", then tell scsi layer that
the device supports runtime power off.

Signed-off-by: Aaron Lu <aaron.lu@amd.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>