[libata] ahci_platform: fix DT probing

The change in commit 904c04fe "ahci_platform: Add the board_ids..."
doesn't work for the DT probing case as platform_get_device_id returns
NULL. Pick the default ahci_port_info in this case.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Cc: Richard Zhu <richard.zhu@linaro.org>
Cc: linux-ide@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>