fix(stpmic1): update error cases return

Use errno values, or the return of called functions, instead of -1.
Correct some MISRA issues, like braces.

Change-Id: If7b53de5cbfb4d2c9979bce0e594dd92bf07a77a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
1 file changed