pinctrl: pinctrl_stm32: Fix warnings when compiling with W=1

pinctrl: pinctrl_stm32: Fix warnings when compiling with W=1

This patch solves the following warnings:

warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
    if (*idx < 0)
             ^
drivers/pinctrl/pinctrl_stm32.c: At top level:
warning: no previous prototype for 'stm32_pinctrl_probe' [-Wmissing-prototypes]
 int stm32_pinctrl_probe(struct udevice *dev)
     ^~~~~~~~~~~~~~~~~~~

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