tools: gpio: fix debounce_period_us output of lsgpio

Fix incorrect output that could occur when more attributes are used and
GPIO_V2_LINE_ATTR_ID_DEBOUNCE is not the first one.

Signed-off-by: Milo Spadacini <milo.spadacini@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>