ASoC: omap-pcm: Initialize DMA configuration

Initialize the dma_slave_config for PCM DMA transfers,
instead of leaving it uninitialized. Keeps previous data on
the stack from giving us invalid values in uninitialized
members of the config structure.

Signed-off-by: Jim Lodes <jim.lodes@garmin.com>
Signed-off-by: J.D. Schroeder <jay.schroeder@garmin.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Mark Brown <broonie@kernel.org>