ata: fix requirements for CONFIG_AHCI_PCI

Building with CONFIG_AHCI_PCI=y and CONFIG_SCSI_AHCI=n leads to

    drivers/ata/ahci-pci.o: in function `ahci_pci_probe':
    drivers/ata/ahci-pci.c:21: undefined reference to `ahci_probe_scsi_pci'

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>