ASoC: stm32: i2s: fix dma configuration

commit 1ac2bd16 upstream.

DMA configuration is not balanced on start/stop.
Move DMA configuration to trigger callback.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>