drivers: gpio: stm32: fix CONFIG_SOC_SERIES_STM32F4X build break

The commit c90e4d063be0ff5f47df45ca8500eff8518c5d25 changed
line 216 in drivers/gpio/gpio_stm32.c:
-elif CONFIG_SOC_SERIES_STM32F4X
+#elif /* CONFIG_SOC_SERIES_STM32F4X */

This causes the following compiler error:
zephyr/drivers/gpio/gpio_stm32.c:218:39: error: #elif with no expression
 #elif /* CONFIG_SOC_SERIES_STM32F4X */
                                       ^

Change-Id: Ie8b124931e333aa7860e6db22f5c259670e29833
Signed-off-by: Michael Scott <michael.scott@linaro.org>