iio: adc: stm32-adc: Fix docs wrongly marked as kernel-doc

W=1 highlights these two cases that are obviously not in kernel-doc
format.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>