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/scsi.c b/cmd/scsi.c
index 5f710d2..4549995 100644
--- a/cmd/scsi.c
+++ b/cmd/scsi.c
@@ -50,7 +50,7 @@
 		}
 	}
 
-	return blk_common_cmd(argc, argv, IF_TYPE_SCSI, &scsi_curr_dev);
+	return blk_common_cmd(argc, argv, UCLASS_SCSI, &scsi_curr_dev);
 }
 
 U_BOOT_CMD(