mfd: rn5t618: Fix IRQ trigger by changing it to level mode

During more massive generation of interrupts, the IRQ got stuck,
and the subdevices did not see any new interrupts. That happens
especially at wonky USB supply in combination with ADC reads.
To fix that trigger the IRQ at level low instead of falling edge.

Fixes: 0c816045 ("mfd: rn5t618: Add IRQ support")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>