iio: adc: stm32: fix scan of multiple channels with DMA

commit 04e491ca9df60ffe8637d00d68e5ab8bc73b30d5 upstream.

By default, watermark is set to '1'. Watermark is used to fine tune
cyclic dma buffer period. In case watermark is left untouched (e.g. 1)
and several channels are being scanned, buffer period is wrongly set
(e.g. to 1 sample). As a consequence, data is never pushed to upper layer.
Fix buffer period size, by taking scan channels number into account.

Fixes: 2763ea05 ("iio: adc: stm32: add optional dma support")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>