After the move of the postenable code to preenable, the DMA start was
done before the DMA init, which is not correct.
The DMA is initialized in set_watermark. Because of this, we need to call
the DMA start functions in set_watermark, after the DMA init, instead of
preenable hook, when the DMA is not properly setup yet.

Fixes: f3c034f6 ("iio: at91-sama5d2_adc: adjust iio_triggered_buffer_{predisable,postenable} positions")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20200923121748.49384-1-eugen.hristev@microchip.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>