gpio: uapi: drop trailing period from one sentence descriptions

Make documentation more consistent by using trailing periods only for
multi-sentence field descriptions, not single sentence descriptions.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>