iio: adc: qcom-pm8xxx-xoadc: Demote standard comment block and supply missing description

iio: adc: qcom-pm8xxx-xoadc: Demote standard comment block and supply missing description

Kerneldoc is only suitable for documenting functions and struct/enums.

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

 drivers/iio/adc/qcom-pm8xxx-xoadc.c:133: warning: Excess function parameter 'PM8XXX_CHANNEL_INTERNAL' description in 'PM8XXX_CHANNEL_INTERNAL'
 drivers/iio/adc/qcom-pm8xxx-xoadc.c:133: warning: Excess function parameter 'PM8XXX_CHANNEL_125V' description in 'PM8XXX_CHANNEL_INTERNAL'
 drivers/iio/adc/qcom-pm8xxx-xoadc.c:133: warning: Excess function parameter 'PM8XXX_CHANNEL_INTERNAL_2' description in 'PM8XXX_CHANNEL_INTERNAL'
 drivers/iio/adc/qcom-pm8xxx-xoadc.c:133: warning: Excess function parameter 'PM8XXX_CHANNEL_MUXOFF' description in 'PM8XXX_CHANNEL_INTERNAL'
 drivers/iio/adc/qcom-pm8xxx-xoadc.c:412: warning: Function parameter or member 'variant' not described in 'pm8xxx_xoadc'

Cc: Andy Gross <agross@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>