gpio: stm32_gpio: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/gpio/stm32_gpio.c: In function 'stm32_offset_to_index':
: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (idx == offset)
            ^~
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/gpio/stm32f7_gpio.c b/drivers/gpio/stm32f7_gpio.c
index 5c9f2fe..e89707c 100644
--- a/drivers/gpio/stm32f7_gpio.c
+++ b/drivers/gpio/stm32f7_gpio.c
@@ -27,7 +27,7 @@
 int stm32_offset_to_index(struct udevice *dev, unsigned int offset)
 {
 	struct stm32_gpio_priv *priv = dev_get_priv(dev);
-	int idx = 0;
+	unsigned int idx = 0;
 	int i;
 
 	for (i = 0; i < STM32_GPIOS_PER_BANK; i++) {