gpio: cdev: update flags at once when reconfiguring from user-space

Make updating the descriptor flags when reconfiguring from user-space
consistent with the rest of the codebase: read the current state
atomically, update it according to user's instructions and write it back
atomically as well.

Reviewed-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20241004-gpio-notify-in-kernel-events-v1-3-8ac29e1df4fe@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>