iio: dac: stm32: fix error message

Fix error message, there's no 'st,dac-channel' property, but 'reg'
(see https://lkml.org/lkml/2017/4/3/567

).

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>