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

drivers/pinctrl/pinctrl_stm32.c

@@ -136,7 +136,7 @@ static struct udevice *stm32_pinctrl_get_gpio_dev(struct udevice *dev,
 			 */
 			*idx = stm32_offset_to_index(gpio_bank->gpio_dev,
 						     selector - pin_count);
-			if (*idx < 0)
+			if (IS_ERR_VALUE(*idx))
 				return NULL;
 
 			return gpio_bank->gpio_dev;
@@ -215,7 +215,7 @@ static int stm32_pinctrl_get_pin_muxing(struct udevice *dev,
 
 #endif
 
-int stm32_pinctrl_probe(struct udevice *dev)
+static int stm32_pinctrl_probe(struct udevice *dev)
 {
 	struct stm32_pinctrl_priv *priv = dev_get_priv(dev);
 	int ret;