pmic: stpmic1: update error cases return

Use errno values, or the return of called functions, instead of -1.

Change-Id: If7b53de5cbfb4d2c9979bce0e594dd92bf07a77a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/161571


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Etienne CARRIERE <etienne.carriere@st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>