iio: dac: ad7303: Complete 'struct ad7303_state' doc

iio: dac: ad7303: Complete 'struct ad7303_state' doc

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

 drivers/iio/dac/ad7303.c:49: warning: Function parameter or member 'vdd_reg' not described in 'ad7303_state'
 drivers/iio/dac/ad7303.c:49: warning: Function parameter or member 'vref_reg' not described in 'ad7303_state'
 drivers/iio/dac/ad7303.c:49: warning: Function parameter or member 'lock' not described in 'ad7303_state'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200716135928.1456727-31-lee.jones@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>