bsec: update bsec_write_debug_conf function type

Checking written value is useless, as local variables.
Set function type to void.

Change-Id: I5ecad094c2c91a2d5a233addbaba5f76c70eab20
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/187859


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