auth: correct sign-compare warning

Correct the warning due to compare signed and non signed variable.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: Ic12527f5f92a34e925bee3047c168eacf5e99d8a
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/189410


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>