iio: dac: ltc2632: Fix formatting in kerneldoc struct header

Kerneldoc expects attributes/parameters to be in '@*.: ' format.

Fixes the following W=1 kernel build warning(s):

 drivers/iio/dac/ltc2632.c:50: warning: Function parameter or member 'powerdown_cache_mask' not described in 'ltc2632_state'
 drivers/iio/dac/ltc2632.c:50: warning: Function parameter or member 'vref_mv' not described in 'ltc2632_state'
 drivers/iio/dac/ltc2632.c:50: warning: Function parameter or member 'vref_reg' not described in 'ltc2632_state'

Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Silvan Murer <silvan.murer@gmail.com>
Cc: Maxime Roussin-Belanger <maxime.roussinbelanger@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>