ASoC: Fix DMA channel leak in imx-pcm-dma-mx2 driver.

"snd_imx_pcm_hw_params" callback can be called
several times by the user (i.e. OSS emulation)
leading to a DMA channel leak.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>