[libata ata_piix] Fix ICH6/7 map value interpretation

The previous change failed to properly mask out unrelated bits, which
resulted in a failure to detect devices.

drivers/scsi/ata_piix.c

@@ -713,7 +713,7 @@ static int piix_init_one (struct pci_dev *pdev, const struct pci_device_id *ent)
 		pci_read_config_byte(pdev, ICH5_PMR, &tmp);
 
 		if (host_flags & PIIX_FLAG_COMBINED_ICH6) {
-			switch (tmp) {
+			switch (tmp & 0x3) {
 			case 0:
 				break;
 			case 1: