The pull field of the line state struct is undocumented. Fix it.

Fixes: 2a9e2740 ("gpio: mockup: rework debugfs interface")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>