ASoC: imx-pcm-dma: open function failed when snd_dmaengine_pcm_open fail

snd_imx_open should return error code returned by snd_dmaengine_pcm_open.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>