libata-acpi: missing _SDD is not an error

Missing _SDD is not an error.  Don't treat it as one.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>