gpio: generic: fix GPIO_GENERIC_PLATFORM is set to module case

GPIO_GENERIC_PLATFORM is a tristate. If the module option is
selected the resulting gpio-generic.ko will lack most of the
module initialzation and probe code.
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-generic.c

@@ -549,7 +549,7 @@ int bgpio_init(struct gpio_chip *gc, struct device *dev,
 }
 EXPORT_SYMBOL_GPL(bgpio_init);
 
-#ifdef CONFIG_GPIO_GENERIC_PLATFORM
+#if IS_ENABLED(CONFIG_GPIO_GENERIC_PLATFORM)
 
 static void __iomem *bgpio_map(struct platform_device *pdev,
 			       const char *name,