ASoC: generic-dmaengine-pcm: Clear slave_config memory

We currently assume that the DMA Slave Config will be fully populated
by the platform, however some DMA Engines make decisions based on zero
(default) flags such as DMA_SLAVE_BUSWIDTH_UNDEFINED and as this is a
static declaration we need to memset it to clear the data area.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>