As every TPM drivers support UCLASS_TPM, we can only rely on DM_TPM
functions.

This simplify a bit the code.

Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>