iio: adc: palmas_gpadc: Demote non-conforming kerneldoc header

iio: adc: palmas_gpadc: Demote non-conforming kerneldoc header

Very few of the struct attributes have been documented here.

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

 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'dev' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'palmas' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'irq' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'irq_auto_0' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'irq_auto_1' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'adc_info' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'conv_completion' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'wakeup1_data' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'wakeup2_data' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'wakeup1_enable' not described in 'palmas_gpadc'
 drivers/iio/adc/palmas_gpadc.c:114: warning: Function parameter or member 'wakeup2_enable' not described in 'palmas_gpadc'

Cc: Pradeep Goudagunta <pgoudagunta@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>