staging: iio: tsl2x7x: correct interrupt handler trigger

tsl2x7x_event_handler() was not called as expected when the device was
asserting a hardware interrupt. This patch changes the interrupt line
trigger from rising to falling.

The driver was tested on a TSL2772 hooked up to a Raspberry Pi 2. The
interrupt pin also had a 10K pull-up resistor per the requirements from
the datasheet. The relevant device tree binding:

&i2c1 {
	tsl2772@39 {
		compatible = "amstaos,tsl2772";
		reg = <0x39>;
		interrupt-parent = <&gpio>;
		interrupts = <22 0x2>;
	};
};

With this patch, iio_event_monitor now shows the events when the
channels are outside the defined interrupt thresholds.

$ sudo ./iio_event_monitor tsl2772
Found IIO device with name tsl2772 with device number 0
Event: time: 1478193460053760446, type: proximity, channel: 0, evtype:
thresh, direction: either
...
Event: time: 1478193463020270185, type: illuminance, channel: 0, evtype:
thresh, direction: either
...

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>