iio: dac: ad5592r: Fix the missing return value.

The third call to `fwnode_property_read_u32` did not record
the return value, resulting in `channel_offstate` possibly
being assigned the wrong value.

Fixes: 56ca9db8 ("iio: dac: Add support for the AD5592R/AD5593R ADCs/DACs")
Signed-off-by: Zizhuang Deng <sunsetdzz@gmail.com>
Link: https://lore.kernel.org/r/20220310125450.4164164-1-sunsetdzz@gmail.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>