[PATCH] ahci: make ahci_fill_cmd_slot() take *pp instead of *ap

Make ahci_fill_cmd_slot() take struct ahci_port_priv *pp instead of
struct ata_port *ap as suggested by Jeff Garzik.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>