iio: dac: ad5504: Fix formatting errors and demote non-compliant kerneldoc

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

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

 drivers/iio/dac/ad5504.c:58: warning: Function parameter or member 'pwr_down_mask' not described in 'ad5504_state'
 drivers/iio/dac/ad5504.c:58: warning: Function parameter or member 'pwr_down_mode' not described in 'ad5504_state'
 drivers/iio/dac/ad5504.c:64: warning: cannot understand function prototype: 'enum ad5504_supported_device_ids '

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>