iio: dac: ad5624r: Drop docs of missing members

The documentation for struct ad5624r_state contains members that are (no
longer?) part of the structure.

Remove unnecessary docs.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/Z2KFC7ZBwmM69Qb4@mva-rohm


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>