ata: ahci_st: remove deprecated struct ahci_platform_data usage

struct ahci_platform_data is deprecated (please see comments in
<linux/ahci_platform.h> for details).  Convert ahci_st driver to
use custom ->host_stop method instead.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>