The TCA8418 might be used using different interrupt triggers on various
boards. This is not working so far because the current code forces a
falling edge trigger.

The device tree already provides a trigger type, so let's use whatever it
sets up, and since we can be loaded without DT, keep the old behaviour for
the non-DT case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>