iio: humidity: hts221: support active-low interrupts

Add support for active low interrupts (IRQF_TRIGGER_LOW and
IRQF_TRIGGER_FALLING). Configure the device as active high or low
according to the requested irq line.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>