pinctrl: pinctrl_stm32: Use GPIOF_UNKNOWN to indicate not mapped pins

GPIOF_UNKNOWN becomes a valid pin muxing information to indicate
that a pin is not mapped.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/drivers/pinctrl/pinctrl_stm32.c b/drivers/pinctrl/pinctrl_stm32.c
index 5d8e156..373f51f 100644
--- a/drivers/pinctrl/pinctrl_stm32.c
+++ b/drivers/pinctrl/pinctrl_stm32.c
@@ -220,8 +220,6 @@
 
 	switch (mode) {
 	case GPIOF_UNKNOWN:
-		/* should never happen */
-		return -EINVAL;
 	case GPIOF_UNUSED:
 		snprintf(buf, size, "%s", pinmux_mode[mode]);
 		break;