Fix to finalize mac compute when there is not a last message chunk.

Code implemented in TAs may need to call TEE_MACComputeFinal,
without last message chunk.

Tested-by: Thierry BINIGUER <thierry.biniguer@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>
Tested-by: Etienne CARRIERE <etienne.carriere@st.com>
Reviewed-by: Etienne CARRIERE <etienne.carriere@st.com>
Signed-off-by: Pascal Brand <pascal.brand@st.com>