gpio: remove export of private of_get_named_gpio_flags()

of_get_named_gpio_flags() has been made gpiolib-private by commit
f01d9075, but its EXPORT statement has not been removed. Fix this.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>