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>