px4nucleo_spi fix sign compare

src/drivers/boards/px4nucleoF767ZI-v1/px4nucleo_spi.c

@@ -230,7 +230,7 @@ __EXPORT void stm32_spi1select(FAR struct spi_dev_s *dev, enum spi_dev_e devid,
 
 	/* Making sure the other peripherals are not selected */
 
-	for (int cs = 0;  arraySize(spi1selects_gpio) > 1 && cs < arraySize(spi1selects_gpio); cs++) {
+	for (size_t cs = 0; arraySize(spi1selects_gpio) > 1 && cs < arraySize(spi1selects_gpio); cs++) {
 		if (spi1selects_gpio[cs] != 0) {
 			stm32_gpiowrite(spi1selects_gpio[cs], 1);
 		}