blk: Switch over to using uclass IDs

We currently have an if_type (interface type) and a uclass id. These are
closely related and we don't need to have both.

Drop the if_type values and use the uclass ones instead.

Maintain the existing, subtle, one-way conversion between UCLASS_USB and
UCLASS_MASS_STORAGE for now, and add a comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/ata/sata.c b/drivers/ata/sata.c
index 0e6c8cd..604c721 100644
--- a/drivers/ata/sata.c
+++ b/drivers/ata/sata.c
@@ -79,7 +79,7 @@
 
 	for (i = 0; i < CONFIG_SYS_SATA_MAX_DEVICE; i++) {
 		memset(&sata_dev_desc[i], 0, sizeof(struct blk_desc));
-		sata_dev_desc[i].if_type = IF_TYPE_SATA;
+		sata_dev_desc[i].if_type = UCLASS_AHCI;
 		sata_dev_desc[i].devnum = i;
 		sata_dev_desc[i].part_type = PART_TYPE_UNKNOWN;
 		sata_dev_desc[i].type = DEV_TYPE_HARDDISK;