gpiolib: fix unwatch ioctl()

Fix the field having a bit cleared by the unwatch ioctl().

Fixes: 51c1064e ("gpiolib: add new ioctl() for monitoring changes in line info")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>