iio: dac: ad5791: Complete 'struct ad5791_chip_info' documentation

... and remove seemingly pointless comment.

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

 drivers/iio/dac/ad5791.c:97: warning: Function parameter or member 'ctrl' not described in 'ad5791_state'
 drivers/iio/dac/ad5791.c:97: warning: Function parameter or member 'pwr_down_mode' not described in 'ad5791_state'
 drivers/iio/dac/ad5791.c:97: warning: Function parameter or member 'pwr_down' not described in 'ad5791_state'
 drivers/iio/dac/ad5791.c:97: warning: Function parameter or member 'data' not described in 'ad5791_state'
 drivers/iio/dac/ad5791.c:103: warning: cannot understand function prototype: 'enum ad5791_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>