ASoC: core: Fix possible NULL pointer dereference of pcm->config

Since the soc generic dmaengine pcm driver allows using the defualt settings,
so the pcm->config maybe NULL.

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>