iio: dac: ad5446: Complete 'struct ad5446_state' doc and demote unworthy kerneldocs

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

 drivers/iio/dac/ad5446.c:48: warning: Function parameter or member 'dev' not described in 'ad5446_state'
 drivers/iio/dac/ad5446.c:48: warning: Function parameter or member 'cached_val' not described in 'ad5446_state'
 drivers/iio/dac/ad5446.c:48: warning: Function parameter or member 'pwr_down_mode' not described in 'ad5446_state'
 drivers/iio/dac/ad5446.c:48: warning: Function parameter or member 'pwr_down' not described in 'ad5446_state'
 drivers/iio/dac/ad5446.c:48: warning: Function parameter or member 'lock' not described in 'ad5446_state'
 drivers/iio/dac/ad5446.c:323: warning: cannot understand function prototype: 'enum ad5446_supported_spi_device_ids '
 drivers/iio/dac/ad5446.c:545: warning: cannot understand function prototype: 'enum ad5446_supported_i2c_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>