Fix GCM Authentication

Authentication tag length, given during the enc_final(),
was wrong. It was the max of the tag buffer, instead of the
value provided during the init.

Other cleanup also takes place to simplify CCM authentication.

Change-Id: I14c02b2a39a51fb485b0ff04707895f91fcc73a3
Reviewed-on: https://gerrit.st.com/12369
Tested-by: Pascal BRAND <pascal.brand@st.com>
Tested-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Reviewed-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Signed-off-by: Pascal Brand <pascal.brand@st.com>