gpio: uapi: comment consistency

Make debounce_period_us field documentation consistent with other fields
in the union.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201005070329.21055-3-warthog618@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>