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/dwc_ahsata.c b/drivers/ata/dwc_ahsata.c
index 1a2c3c2..167b5a3 100644
--- a/drivers/ata/dwc_ahsata.c
+++ b/drivers/ata/dwc_ahsata.c
@@ -880,7 +880,7 @@
 	device_find_first_child(dev, &blk);
 	if (!blk) {
 		ret = blk_create_devicef(dev, "dwc_ahsata_blk", "blk",
-					 IF_TYPE_SATA, -1, 512, 0, &blk);
+					 UCLASS_AHCI, -1, 512, 0, &blk);
 		if (ret) {
 			debug("Can't create device\n");
 			return ret;