[PATCH] ahci: separate out ahci_fill_cmd_slot()

Separate out ahci_fill_cmd_slot() from ahci_qc_prep().
ahci_fill_cmd_slot() can later be used to issue non-standard commands.
(e.g. softreset)

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