pinctrl: nomadik: hide nmk_gpio_get_mode when unused

nmk_gpio_get_mode is only used in one place, and that is conditionally
compiled if DEBUG_FS is enabled. A recent cleanup has marked the
definition 'static', which now leads to a warning:

drivers/pinctrl/nomadik/pinctrl-nomadik.c:614:12: error: 'nmk_gpio_get_mode' defined but not used [-Werror=unused-function]
 static int nmk_gpio_get_mode(struct nmk_gpio_chip *nmk_chip, int offset)
            ^~~~~~~~~~~~~~~~~

Moving the function itself inside the #ifdef shuts it up again.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5e81e0a0 ("pinctrl: nomadik: use BIT() with offsets consequently")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>