gpiolib: Do not mention legacy GPIOF_* in the code

We are going to remove legacy API from kernel, don't mention
it in the code that does not use it already for a while.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>