drivers: it8xxx2: fix GPIO interrupt enables

gpio_pin_interrupt_configure asserts that one of GPIO_INT_ENABLE or
_DISABLE is specified by the caller, and also that GPIO_INT_EDGE is
requested if both states (GPIO_INT_TRIG_BOTH) should interrupt. This
change corrects the misuses in it8xxx2 drivers that cause assertion
failures.

When assertions are disabled the existing code works correctly because
the it8xxx2 GPIO driver assumes that a pin interrupt should be enabled
if _DISABLE is not requested, and the driver only supports edge
triggers but assumes the absence of GPIO_INT_MODE_LEVEL indicates
an edge trigger was requested.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Change-Id: I1aaee190ec4cf063f36e25c0c293a91d280e71bb