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/pvblock.c b/cmd/pvblock.c
index 56ce8b1..1b604c3 100644
--- a/cmd/pvblock.c
+++ b/cmd/pvblock.c
@@ -14,7 +14,7 @@
 
 int do_pvblock(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 {
-	return blk_common_cmd(argc, argv, IF_TYPE_PVBLOCK,
+	return blk_common_cmd(argc, argv, UCLASS_PVBLOCK,
 			      &pvblock_curr_device);
 }