'ret' may be 0 so, dev_err_probe() should be called only when 'ret' is
an error code.

Fixes: 41c9c06c ("mfd: stmfx: Simplify with dev_err_probe()")
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>