pinctrl: ralink: rt2880: fix '-Wmissing-prototypes' in init function

Kernel test robot reported the following warning:
'warning: no previous prototype for 'rt2880_pinmux_init''.
This function is the entry point for the platform driver and
it is private to this driver. Hence declare it 'static' which is
the correct thing to do fixing also this warning.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20201228064727.30098-1-sergio.paracuellos@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/ralink/pinctrl-rt2880.c

@@ -361,7 +361,7 @@ static struct platform_driver rt2880_pinmux_driver = {
 	},
 };
 
-int __init rt2880_pinmux_init(void)
+static int __init rt2880_pinmux_init(void)
 {
 	return platform_driver_register(&rt2880_pinmux_driver);
 }