iio: adc: qcom-vadc-common: fix vadc_scale_fn_type kernel-doc

iio: adc: qcom-vadc-common: fix vadc_scale_fn_type kernel-doc

Fix multiple warnings in enum vadc_scale_fn_type by adding a leading
'@' to the kernel-doc descriptions.

Fixed 14 warnings in this one enum, such as:
Warning: include/linux/iio/adc/qcom-vadc-common.h:123 Enum value
 'SCALE_DEFAULT' not described in enum 'vadc_scale_fn_type'
Warning: ../include/linux/iio/adc/qcom-vadc-common.h:123 Enum value
 'SCALE_THERM_100K_PULLUP' not described in enum 'vadc_scale_fn_type'
Warning: ../include/linux/iio/adc/qcom-vadc-common.h:123 Enum value
 'SCALE_PMIC_THERM' not described in enum 'vadc_scale_fn_type'

Also prevent the warning on SCALE_HW_CALIB_INVALID by marking it
"private:" so that kernel-doc notation is not needed for it.

This leaves only one warning here, which I don't know the
appropriate description of:
qcom-vadc-common.h:125: warning: Enum value
 'SCALE_HW_CALIB_PMIC_THERM_PM7' not described in enum 'vadc_scale_fn_type'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>