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/nvme.c b/cmd/nvme.c
index e715c57..09d5f43 100644
--- a/cmd/nvme.c
+++ b/cmd/nvme.c
@@ -28,7 +28,7 @@
 		if (strncmp(argv[1], "deta", 4) == 0) {
 			struct udevice *udev;
 
-			ret = blk_get_device(IF_TYPE_NVME, nvme_curr_dev,
+			ret = blk_get_device(UCLASS_NVME, nvme_curr_dev,
 					     &udev);
 			if (ret < 0)
 				return CMD_RET_FAILURE;
@@ -39,7 +39,7 @@
 		}
 	}
 
-	return blk_common_cmd(argc, argv, IF_TYPE_NVME, &nvme_curr_dev);
+	return blk_common_cmd(argc, argv, UCLASS_NVME, &nvme_curr_dev);
 }
 
 U_BOOT_CMD(