ASoC: dmaengine_pcm: Reset pointer position when starting a stream

Otherwise a wrong position will be reported after restarting a stream and the
first few samples might be skipped.

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>