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/virtio/virtio_blk.c b/drivers/virtio/virtio_blk.c
index 3ff74f4..9710b79 100644
--- a/drivers/virtio/virtio_blk.c
+++ b/drivers/virtio/virtio_blk.c
@@ -75,14 +75,14 @@
 	struct blk_desc *desc = dev_get_uclass_plat(dev);
 	int devnum;
 
-	desc->if_type = IF_TYPE_VIRTIO;
+	desc->if_type = UCLASS_VIRTIO;
 	/*
 	 * Initialize the devnum to -ENODEV. This is to make sure that
 	 * blk_next_free_devnum() works as expected, since the default
 	 * value 0 is a valid devnum.
 	 */
 	desc->devnum = -ENODEV;
-	devnum = blk_next_free_devnum(IF_TYPE_VIRTIO);
+	devnum = blk_next_free_devnum(UCLASS_VIRTIO);
 	if (devnum < 0)
 		return devnum;
 	desc->devnum = devnum;