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/cmd/sata.c b/cmd/sata.c
index 76da190..9c9fe11 100644
--- a/cmd/sata.c
+++ b/cmd/sata.c
@@ -27,7 +27,7 @@
 	struct udevice *dev;
 	int rc;
 
-	blk_unbind_all(IF_TYPE_SATA);
+	blk_unbind_all(UCLASS_AHCI);
 
 	rc = uclass_find_device(UCLASS_AHCI, devnum, &dev);
 	if (!rc && !dev)
@@ -111,7 +111,7 @@
 		sata_curr_device = 0;
 	}
 
-	return blk_common_cmd(argc, argv, IF_TYPE_SATA, &sata_curr_device);
+	return blk_common_cmd(argc, argv, UCLASS_AHCI, &sata_curr_device);
 }
 
 U_BOOT_CMD(