staging:iio:ad7291: Use event spec for threshold hysteresis

Register the event threshold hysteresis attributes by using the new
IIO_EV_INFO_HYSTERESIS event spec type. This allows us to throw away a good
portion of boiler-plate code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>