ata: ahci_platform: convert kzallloc to kcalloc

It's better to kcalloc instead of kzalloc(n * sizeof())

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