ASoC: atmel_ssc_dai: move set dma data to startup callback

move set dma data to startup callback function, if the set dma
data exist in hw_params callback, so the dma data only usable when
call hw_params, if want use it before hw_params callback, it will
cause NULL pointer access oops

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>