ata: ahci_st: build fixes

* The config option for ahci_st driver was renamed from
  CONFIG_SATA_AHCI_ST to CONFIG_AHCI_ST but Makefile was
  not updated.  Fix it (also while at it move the ahci_st
  driver entry below ahci_imx and ahci_sunxi ones).

* Fix a few build issues in the ahci_st driver itself.

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