plat/st: Fix incorrect return value

Change the return code in boot_api.h which impacts the
authentication result.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I2c3aadb98dd261ae5ad73978fc74a8a8cfa59b82
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
1 file changed