iio: adc: stm32-dfsdm: fix multiple channel initialization

When several channels are registered (e.g. via st,adc-channels property):
- channels array is wrongly filled in. Only 1st element in array is being
  initialized with last registered channel.
  Fix it by passing reference to relevant channel (e.g. array[index]).
- only last initialized channel can work properly (e.g. unique 'ch_id'
  is used). Converting any other channel result in conversion timeout.
  Fix it by getting rid of 'ch_id', use chan->channel instead.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>