gpio: stmfx: rename function used to change pin configuration

Rename the two function used to change the pin configuration
from stmfx_pinctrl_.. stmfx_conf_... to clarify the function usage.

No functional change.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>