ata: ahci_platform: convert kcalloc to devm_kcalloc

Like phys, target_pwrs could be allocated with devm_ function

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>