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/usb.c b/cmd/usb.c
index 3d87376..2ba0569 100644
--- a/cmd/usb.c
+++ b/cmd/usb.c
@@ -719,7 +719,7 @@
 	if (strncmp(argv[1], "stor", 4) == 0)
 		return usb_stor_info();
 
-	return blk_common_cmd(argc, argv, IF_TYPE_USB, &usb_stor_curr_dev);
+	return blk_common_cmd(argc, argv, UCLASS_USB, &usb_stor_curr_dev);
 #else
 	return CMD_RET_USAGE;
 #endif /* CONFIG_USB_STORAGE */