iio: dac: ltc2632: drop some duplicated data

The channel data for ltc2632l12 and ltc2632h12 are identical. So there
is no gain in using two different (but identical) channel descriptions.
The only side effect of this change is some memory savings.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>