gpio: staticize xway_stp_init()

This initcall is only called from the driver itself, staticize it.
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-stp-xway.c

@@ -292,7 +292,7 @@ static struct platform_driver xway_stp_driver = {
 	},
 };
 
-int __init xway_stp_init(void)
+static int __init xway_stp_init(void)
 {
 	return platform_driver_register(&xway_stp_driver);
 }