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/include/mmc.h b/include/mmc.h
index f519d86..027e8bc 100644
--- a/include/mmc.h
+++ b/include/mmc.h
@@ -767,7 +767,7 @@
 /**
  * mmc_bind() - Set up a new MMC device ready for probing
  *
- * A child block device is bound with the IF_TYPE_MMC interface type. This
+ * A child block device is bound with the UCLASS_MMC interface type. This
  * allows the device to be used with CONFIG_BLK
  *
  * @dev:	MMC device to set up