libata-acpi: miscellaneous cleanups

* Add missing LOCKING: and RETURNS: to function comment.

* Don't conditionalize warning messages with ata_msg_probe().  Print
  directly with KERN_WARNING.

* Drop duplicate debug messages.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>