libata-acpi.c: fix copy and paste mistake in ata_acpi_register_power_resource

Fix a copy and paste mistake introduced in:

commit bc9b6407
"ACPI / PM: Rework the handling of devices depending on power resources"

Signed-off-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>