ASoC: dmaengine: Add config parameter NULL check.

Because the "ASoC: dmaengine-pcm: Provide default config" has provided
us one defualt config of DMA. When using this, the config parameter of
devm_snd_dmaengine_pcm_register() will be NULL, so here we need to have
a check before using it.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>