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

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>