gpiolib: cdev: clear debounce period if line set to output

When set_config changes a line from input to output debounce is
implicitly disabled, as debounce makes no sense for outputs, but the
debounce period is not being cleared and is still reported in the
line info.

So clear the debounce period when the debouncer is stopped in
edge_detector_stop().

Fixes: 65cff704

 ("gpiolib: cdev: support setting debounce")
Cc: stable@vger.kernel.org
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>