staging:iio:cdc:ad7150: Simplify event handling by only using rising direction.

The event line is active high and not maskable within the device.
It indicates current state directly.

The device supports separate rising and falling thresholds so rather
than trying to using each bound to detect in both directions just use
IRQF_TRIGGER_RISING.  If a user wants to detect the value falling
back below the threshold, then set the falling threshold appropriately.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-8-jic23@kernel.org