ASoC: imx-pcm-dma: check kzalloc return value in function snd_imx_open

It fixed smatch warning:
sound/soc/fsl/imx-pcm-dma.c:112 snd_imx_open() error: potential null dereference 'dma_data'.  (kzalloc returns null)

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